Look.... FORGET about the "power" rating on the back of (or wherever it is) the speaker. It's basically meaningless. When a speaker is rated at 90 dB with a single watt of power (or whatever the "efficiency" or "sensativity" rating of the speaker is), it typically takes double the power to raise the output by 3 dB. So, at 95 dB, the power going into a speaker rated at 90 dB, would be 16 watts of power. The 90 wpc on your receiver is the power available. Not the power it produces on a regular basis.
The "power rating" on the back of the speaker has absolutely nothing at all to do with how loud it plays. The efficiency rating (shown as a dB rating), will give you an indication of how loud a speaker will play at a certain power level. If you are mis-matching speakers across the front, when you do your speaker calibration, either through the receiver's own calibration software or by doing it manually using an SPL meter (my own personal preferred method), this will take into account the different levels of output from the speakers and bring them all to the same SPL's (sound pressure levels).
About the only good thing the power rating on a speaker is good for, is to tell you the maximum amount of power it can handle on a sustained basis. So, using the 25 watts you mentioned (and just guessing on the efficiency rating of the speakers.... the dB rating), based on an efficiency rating of 90 dB's, you would need to be at roughly 96 dB's sustained (32 watts of power being applied to the speaker), before they would most likely start to break up and start to fry. To give you an idea, 96 dB's sustained
is loud enough to cause permanent hearing loss. http://www.gcaudio.com/resources/howtos/loudness.html
As to hooking up the speakers, I'm still not clear what speakers you have. I've gathered that you have some HTIB Onkyo speakers. If you do, then throw them away and forget about them IMHO. They were designed to be paired with your HTIB receiver which you've obviously replaced with the new(er) 605. If you simply must use them right now, fine, but doubling them up on any channel, for whatever reason, is not an option. IMHO
This is totally mind blowing.... these statements are more confusing than I am....
1. I can connect one to the center posts on the onkyo 605, but that leaves the other dangling and, 2. I'm assuming, the center channel output leaning left/right depending on which I have connected. 3. I can just ignore the other side, double them up so I get (lower powered) balance or 4. re-assign one of my unused rear surrounds to center channel (can I do that?). 5. The only benefit of doubling the speakers on the center channel post would be to spread that center audio rather than leave it lopsided.
Trying to straighten this out and make some sense of it....1. Leaves what other "DANGLING"? There are speaker outputs for the right front channel, center channel, left front channel, right rear surround channel, left rear surround channel (there are obviously more, but for this situation, that's enough). The signal for EACH channel is independent of the others. Nothing is left "dangling." If you don't connect a speaker to the output, you get no signal for that channel period. If that's what you mean by dangling, then you are correct. No speaker connected, no sound for that channel. Simple as that.2.
Nothing is left "leaning." Either you connect a speaker to the binding posts on the receiver and get sound.... or you don't. The center
channel sound doesn't magically lean one way or the other if no right front speaker is connected to the right front binding posts on the receiver. You simply get zero sound from the un-connected channel. No leaning involved sorry.3.
Those first two were relatively easy, these next questions are bizarre, so forgive me ahead of time here.... by "ignoring" the other side, I have to assume you mean ignoring the front left or front right channels. If you also by ignore, you mean that you don't connect any speakers to the front right or front left channel outputs..... you will not get ANY sound from a channel that you don't connect a speaker to. "Double them up".... I'm assuming you're back to doubling up the center channel speakers here.... but this baffles me.... double them up so I get (lower powered) balance....
I'm thinking that you mean that buy somehow using two (lower power rated?) speakers on the center channel output rather than one center channel and one right front channel and NO left front channel (leaving it dangling), you will somehow get a more "balanced" sound? If that's what you are thinking.... forgetaboutit, no nyet, nonka. As I've already said (but well worth repeating), you will get sound from the channel you have speakers connected to period.4.
The un-used rear surround channels will not magically give you center channel encoded information by placing a rear surround connected speaker under the TV. You will get sound that's encoded for that channel. As for re-assigning it to a center channel, first of all, that's not possible. But, I'm still INCREDIBLY confused as to why you'd even want to. You have a center channel connection.... USE it.5.
There is absolutely NO benifit to doubling up and connecting two speakers to your center channel output on your receiver. If you don't have a good place for your center channel speaker (round edges, egg shaped or otherwise), simply connect NOTHING to the center channel speaker outputs. Then in your receiver speaker set up menu, tell the receiver you have no center channel speaker (often times called phantom center). The receiver will then split the center channel signal equally between the R/L front main channels. This is the ONLY option you should be considering as near as I can figure out from your post.
I hope this cleared up the foggy state of confusion. Maybe if you tell us exactly what speakers you have that work, and then we can tell you the best way to configure your rig. Don't try to make this more confusing than it is. It's really pretty simple if you stick to the basics.