Audio theory on "upgrade or expand".

Decide on 3.0 (or whatever) over 2.0 depending on your usage criteria - if you're just watching movies (that are 5.1 encoded) then 3.0 can make more sense, you end up with a center channel; if you're purely listening to music or other stereo sources, 2.0 makes sense.

Adding a subwoofer is based on desired/needed bass response in either situation - with pure music listening you may not find it needed if the speakers are suited to low end response (And they don't have to cost a fortune to do this); it's really preferential.

Based on your stated usage criteria, and your budget, I would probably suggest stereo speakers and a multi-channel AVR, add a center channel in if you feel that it would be beneficial (they're usually much cheaper than the main L/Rs from a series, like the Yamaha Soavo - $4000+ for the main pair, and ~$900 for the center), same for a sub.

Built in flexibility and less likelihood of obsolescence. Do you want to go to a home theater system as in 5.1 or preferably 7.1? If not, then you have to consider the capabilities of your existing fronts, relative to a sub.

Music rarely makes it under 40hz. Unless you have a penchant for pipe organ music or the 1812 overture performed outdoors with an actual cannon(which I have such a copy).

Probably 90% of music(this included rap) stays above 60hz. I would purchase a good PAIR of speakers capable under 50hz. Then get the appropriate center.

Unless your movie watching is action(Gamer/Transformers/GI Joe), you won't miss not having a subwoofer.

I watch a lot of foreign films(the wife and I over the weekend watched the Three Colors Trilogy again, good movies). I don't think the subwoofer in the room even made a peep all weekend.

Excellent movies...although "White" doesn't stand on its own.
http://www.amazon.com/Three-Colors-T...9188848&sr=1-2

Pipe organ music and 1812 canons get you down to 16 Hz or lower. All you need to get to a mere 32 Hz is a tuba a harp, or a 5-string bass.

http://www.independentrecording.net/...in_display.htm

Ok, how much "top 40" uses...

Harp? (Florence and the Machine, you got me)
Tuba? (The rare weirdness on American Idol doesn't count)
5 string bass? (Seven Nation Army doesn't even get that low)
Cannons? (really???)
Pipe Organ? (when was the last time pipe organ music cracked the top 10,000?)

Here is what I learned:

1. More speakers will almost always sound better than less speakers since additional speakers will almost always provide with better envelopement.
2. Front speakers will almost never handle low frequencies as good as a separate subwoofer due to their size limitations. You can take an exception to this for $100K speakers that need a forklift to be moved, but even then a separate subwoofer is preferable because it provides you placement flexibility. Where you need to place front speakers is hardly ever the best location for the subwoofer.

Conclusion: Definitely 3.1

You haven't lived until you've heard a tuba & pipe organ call-and-response jam finished off with a cannon shot.

hd_newbie above touched on the correct answer. Since you are asking for the theory, I'll start by simply stating the cliffnotes answer as above and explain later as I have time if you don't find them after researching the topics or someone else doesn't elaborate further first.

Sub is almost universally preferred for a few reasons. Placement for optimum sub response is almost never the same as optimum for left and right speakers. They are placed for stereo separation, imaging, speculat reflections, and sbir considerations. Subs are placed for smoothest modal response, and their sbir placement requirement is diametrically opposed to that of the mains because they are on the other side of the xo point. Response down to 20hz or lower is required even for music. Transients of higher pitched instruments such as attack of a piano hammer or pick of a guitar string or strike of a drumstick have very wide bandwidths. Loss of the low frequency component robs music of it's realism.

Three channels with capable processor are preferrable to stereo l/r even for stereo source material. A good processor will do a better job of processing the signal than your room ever will, which is what you are relying on with just two speakers. For this reason 5 are even better, seven better still... etc.

Some of the above may be controversial but all is based on sound science and decades if research and experience.

Explaining any of the topics above would require a book in itself, but after you search for and read more about these issues we can answer more specific questions.

Can you explain how adding a center channel to stereo reproduction enhances envelopment?


I agree subwoofers are better than none.


Not in my experience, where the center channel provides a visual (and audible) anchor to depth perspective, in what is supposed to be a 3D scene...if your LR loudspeakers are capable of producing sufficient depth, which can place "center" images substantially rearward of the LR plane. YMMV.

cheers,

AJ

Could you cite some research references where 3ch was preferred to stereo with stereo material? TIA.

cheers,

AJ

Who said stereo? He said 70% music and 30% movies. Music can be debatable, but one can certainly benefit from adding center channel for movies.

If you have only stereo imaging, the center will drift as you move laterally to the MLP because it is a phantom image. With a center speaker, the apparent center is rock solid as you move laterally. That is the difference. Whether or not it is more enveloping is debatable. But at least anyone, as in guests, will hear the audio image properly if they are sitting off axis. That is the trap of two channel/speaker reproduction. If you only sit in the MLP, it is fine. But should one move off axis and dramatically so, then the phantom center disappears.

Don't underestimate what a modern AVR can do with music in your room. Bass management if you want to add a sub or two... Add another speaker or two or three or four or five... Room EQ... Setting distance/time delay for each speaker... Being able to decode modern HD formats. Many AVRs have pre-outs for all channels so you can add as much power as you see fit down the road as your system grows.

Say you go from 2.0 to 3.0 and you like how you can set your L/R speakers further apart for a wider soundstage yet still have sharply focused center vocals. Then you try a sub and like the more sharply defined bass. Then you add a set of bookshelve speakers to become a 5.1 system and you like the wider and deeper soundstage again!

And realize an AVR can take a 2ch source and see what sounds are meant to be center or to the sides of the soundstage and distribute them accordingly between as many speakers as your system has for a soundstage as wide and deep as you choose.

For me, playing back with 2 speakers only seems a bit dull now.

The OP:

To which you responded:

So I'll ask again. How does adding a center (and/or sub) almost always provide better envelopment?

That I agree with.

cheers,

AJ

Right. If you read what I wrote, that's what I'm questioning.

cheers,

AJ

Like the other poster said, without center, the OP needs to sit in the dead center of the 2 speakers. Phantom center is hardly recommended for movies, which you agree with and that was my point anyway.

For music, it is more debatable. But still remember that many SACDs are recorded in 5.1.

Note: OK, I just realized you got stuck on my comment on "enveloping". i guess I may not have used the appropriate terminology. However, this would not change my conclusion that 3.1 is a better set up than otherwise.

The decision is what you prefer.
For 70% music, IMHO, the bulk of the budget goes towards mains (LR), the rest on subs and center....depending on a lot of other factors.
Size room, furnished, where can the speakers be placed...relative to where you sit...and how many people typically sit there with you, etc, etc.

cheers,

AJ

Your guess is correct . That was my only contention.


For movies, I agree...especially for multiple viewers (not mentioned by the OP unless I missed it).

cheers,

AJ

No.

As in, answer questions asked of you first and then I might actually consider returning the courtesy.

TIA.

....Nothing.
Yep, agreed.

cheers,

AJ

As in I won't. As I don't care what the question is. As in try badgering someone else with your incessant questions.

Can you cite some research showing preference for two vs more channels when combined with competent processor for two channel sources?

TIA

True.
True
They win on mids and highs.
Personally I would always upgrade sub first.
No, you are on money.

You have limits on what you think that you can afford to spend now. That is the way that it is. And no, it doesn't make any sense to buy a $1k sub when the rest of your audio system doesn't even cost that. Find a sub that is more proportional to whatever your system happens to have cost. Think of this as a step in your audio evolution, meaning that you could easily end up junking what you have now when you are willing to spend more for something that, hopefully, is a lot better. But you may never come around to that point and be satisfied with what you have. And there is nothing wrong with that. Upgrading your fronts, whatever, don't think that that is the answer. Think carefully about what you may want to do down the line. IMHO, the worst thing that could do would be to buy full range fronts and then have to integrate them into a home theater system with a sub. Better yet, go out and listen to others home theater systems and see what is possible.

SBIR (speaker boundary interface response) is a phenomenon by which a null is created at a specific frequency that is dependent on a speaker's location in proximity to a hard boundary (wall, floor, ceiling). It is the frequency at which the speaker's reflection off the boundary is out of phase with the speaker itself due to the total distance (to wall and back) traveled being half the wavelength at the null frequency.

Moving a speaker further from a wall lowers this frequency. Closer to a wall (or floor, or whatever) raises it. You generally want mains spaced far enough from the wall so that the null frequency is below the crossover point. You generally want the sub close enough such that the null is above the crossover point.

xo is shorthand for crossover. In this specific case, it can be simplified well enough as the frequency which divides mains and sub. Thus the requirements of moving sub closer to and mains further from the wall are mutually exclusive when sub and mains are located in the same physical enclosure.

Thus having "full range" mains try to cover the lowest frequencies as well is "wrong" from this theoretical perspective, and in practical terms makes it much more difficult to get even response for a generous seating area. This applies whether the mains are $50, $500, or $50,000.

So you really should always have a sub, for music, movies whatever. Not only because a separate sub is typically more capable, having an active crossover, more volume displacement, etc. but because as above it's simply the correct way to go about reproducing the full spectrum in a small room. Care must be taken in properly integrating the sub in level, phase, and crossover characteristics with the mains, and this can sometimes be a bit tricky depending on the construction of the mains. Advice here is to pick properly designed mains and life is much easier. A flexible receiver makes this much easier as well.

As for the center, I'd place its importance after a sub, both for movies and music. I think it is crucial for best reproduction of both, but no more important than a sub.

As for cost, you have to start somewhere. It makes no sense to "waste" $500 or so on a cheapish underperforming sub that you will then have to replace once you upgrade mains and/or add a center and/or surround channels. I'd suggest a quality sub, diy or commercial, that will be at home in your system now and in the future.

You can hypothesize and "generally speak" until you are blue in the face and still get nowhere.
Btw, I looked up your speakers.
For a 70% music, 30% movies guy, by far the biggest improvement for your $$ will be a new set of mains. You mentioned a budget, but zero about where you are (US?), room size, arrangement, speaker placement relative to seating, etc, etc. all of which must be considered.
Let's delve into the real world for a minute.
If your budget is $1500ish, I would highly recommend auditioning the highest performance mains in the $1000-1200 range, the rest on center and sub. By highest performance I mean soundwaves, which are measurable, not psychogenic preferences.
Two real world examples:
Revel F12's $1010/pr
KEF Q900 $1100/pr.
Both offer superior off axis response to the typical tragic box designs (like your current speakers)...and thus will sound good in a wide variety of rooms...without the need for resorting to stuffing the room full of bandaids/gauze "treatments". Both are sensitive enough to get quite loud with your current Onkyo...and play plenty deep (below 30hz) in room for music to where a sub will not be immediately a dire need.
Another $150 (or a small vertical bookshelf if possible) - $280 gets you a "closely enough" matched center.
Either way, you're still under $1500, so you can start thinking subs...and will have a clearly audible improvement (should be rather obvious) in sound.
Unless your room is a 8x8 tiled bathroom.
Don't forget, just because you have a center for movies, doesn't mean you can't switch to 2.0 (stereo) for music...which I would recommend.
As for needing to spend at least $1000 for a sub...

cheers,

AJ

Still waiting for a response.

There are two channel amps because there is a market for it and it maybe needed. For example, I have a 9.2 system that is completely supported by the AVR (9 separate amps for the 9 speakers), but it can go to an 11.2 system which would then need an external amp for the other two channels.

As to 2 ch audio, that can be highly contentious and some swear up and down about it. So there is a niche for it, but I have no interest in it, but you can usually run a 5.x system in direct (2 channel and no room correction) or stereo mode (with room correction). So call it personal preference.