Looking for some help...
I have some old, but large (bassy) front speakers-Infinity SM152's. (Not sure if this are good speakers, but they are loud!!). I just bought a new Onkyo 605 7.1 receiver.
Do I need to buy a seperate sub for the 7.1? The front speakers come with 15 inch woofers already. My question is with a 7.1 hookup--is the low frequency set to come out of the sub output?

Thanks

Also looking for any advice on a good center to go with these.

In the 605 go to the speaker settings and set the sub to "no" and the right and left speakers to "large". Now listen to them. Next is the really hard part, because now you have to decide if you need a sub.

yes u need a sub

Yes, you need a subwoofer. No matter what kind of woofer is in your speaker, it is not going to produce the same impact of a real subwoofer. Even with your 15 inch, it's not going to reproduce even the impact of a well designed 10'' subwoofer. The receiver isn't giving enough power to the speaker's woofer to give that room shaking bass. There's probably not enough enclosure volume for the driver to reproduce deep bass. The closest speaker I've heard that reproduced deep bass with good impact is the Definitive Technology BP7000SC. They have an 1800 watt internal amplifier for their woofer, so they get the power to reproduce it. But other than that speaker, I've never heard a speaker that can reproduce deep bass with authority.

Towers are just not going to do LFE like a sub will, with the same "all around and through you" effect.
And since you have good fronts you will need to get a good sub to compliment them.
Your fronts will also benefit from the power they'll be able to use that wont have to go all to bass, with a powered sub.

In the 605 go to the speaker settings and set the sub to "no" and the right and left speakers to "large". Now listen to them. Next is the really hard part, because now you have to decide if you need a sub.

If you are satisfied with the bass that you can get with your speakers, you do not need a subwoofer. If you turn up the bass on your receiver, you might be satisfied. If turning up the bass does not work for you, then a subwoofer will be needed.

The X Series center channel from AV123 is a good way to go.

Um...you're in the Subwoofer subforum, most of us will say "YES, you need a sub!". :D

To answer your question, yes, you need a sub. I didn't know anything about your Infinity SM152s and looked them up. Frequency response not low enough. :D

Here's a link of someone else with the exact same front speakers I have --Infinity SM152's. They are huge and outdated, but very loud with deep bass. I'm not sure if I bought an new expensive pair of fronts and sub that it would sound much better than these old things.

http: / /atlanta.craigslist.org/ele/518242943.html

Here are the specs. I'm not an expert on the quality of these by looking at the specs.

PRODUCT SPECIFICATIONS Infinity SM 152

General

Woofer
15" (381mm) polypropylene coated

Power Rating
10 - 300 watts

Frequency Response
44Hz - 25kHz (+/-3dB)

Midrange Drivers
TWO - 4.5" (114mm) polypropylene coated

Tweeter
1" (25mm) high-output polycell

Nominal Impedance
8 ohms

Sensitivity
102dB (1 watt/1 meter)

Crossover Frequency(ies)
500Hz, 5.5kHz

Wow, those are efficient. 102 db!

But loud isn't everything. And since it's already 3 db down at 44 hz, no matter what kind of crossover slope it has, there will not be significant output near 20 hz, and certainly not below it. Unless you listen to stuff with no low bass, you need a sub. Good news is that the sub can probably focus on freq well below 80hz. You can experiment with crossover points in your receiver with more flexibility than most people.

This further proves my point. 44Hz is not even near the requirement for deep bass. You definitely need a subwoofer.

The large speakers in the center of this page are what i own.
http://www.polksda.com/
Incredible sound but no punch. The Polks go deep but...
I didn't realize how much a sub would change my room until this week. Just bought a Epik Castle.
Could have gone with the Knight or Caliber, but wanted to be darn sure I would be happy.
I spent all day listening to long forgotten cds. They all have new life now.
Good luck on your decision.

To most of the guys in this thread 44Hz is not low. In fact you will find a few guys here that use a crossover point near 44HZ. I know that several use 50Hz for their crossover. I think that you will be shocked at the difference a good sub will make to your system.