What size subwoofer to get?

As I concur with the above advice I would suggets the Vandersteen Sub which used should run $750.It has an execllent crossover which is is accessed through a direct hook up to your speaker posts and responds to the full range signal not just what is low passed.Very clean and quick do also to fact that it uses multiple 8" drivers.Call a sub manufacturer and they will tell you that bigger is better but a large cone and magnet assembly does not always move fast enough to reproduce pitch accurately.If it were Home Theatre thast would be a different story where bigger is better for that "boom in the room".I vote for Vandersteen and if you can afford it the REL Strata III (starting with one and getting second).But all mentioned above are good.You didn't say what kind of assciated equipment you had so all of us are being presumptuous and as you will have noticed freaks.But as it was mentioned before a really cheapo sub good for HT will screw up decent sound.If you are looking try to find one with smaller faster cone and get two but also try to look for one with a servo amp which will produce less distortion than a non-servo.
G'luck

Gunbei: It's a good question, but fortunately you almost never need to compromise. The flatest response is with subs placed on opposite walls at the midpoint. Now if it were for music and it was a high crossover point (say 80Hz or higher) I would move them back closer to the main speakers but symetrical, and relatively close to the wall (within 1/8 of a wavelength of the crossover point). In a non-symetrical room, this may not be the case. Here's an even more interesting phenomenon (purely hypothetical--because I don't know anyone that's actually done a room like this). If you use 4 subwoofers and place them 1/4 of the crossover wavelength from the corners of the room you get a nearly flat response throughout the room--no peaks or nulls (obviously there would be minor ripples due to furniture and other items in the room in practice, but in theory it's a flat response). In this last example it does assume a mono signal. In your case, you would not need a mono signal because most of the lower bass notes are mono. If the crossover point is high--then that might not be true and you could run into some difficulties, but no more than having 2 main channel speakers that carry the bass information independently. So, I guess I don't really see any significant downside to stereo subs--I hope I've explained that adequately.

Thanks Rives, give a guy with two subs an excuse to buy two more...

Thanks for the helpful info, my system can be viewed by clicking system link by my name, my room is 14x22. I have PSB Silver-I and wanted a sub for 2 channel listening only as an alternative to upgrading to PSB Gold-I. As stated by others here I don't want to muck up sound with slow, boomy sounding sub.
I naturaly was looking at PSB powered subs which are quite affordable and discounted heavily on internet.

The bass from the Silvers is almost enough but needs a touch more extension and authority. I am looking seriously at 10 or 12 inch PSB powered sub which on paper gives equal or greater bass vs Gold-I, but was wondering about the difficulty in general of having seamless integration of sub with Silver so sound is not compromised.

If I get two subs then the cost is approaching the Gold-I, plus don't want all the clutter and connection hassle of two subs.......can the Silvers sound good with one PSB powered sub?

Thanks very much for the response Rives. If I were to use two stereo subs set up on opposite SIDE walls would that be an example of what you're suggesting? Does it matter if they are downfiring or front firing aimed at each other? Because of space limitations I currently have a single mono sub on one of the side walls, but I'm finding that my sitting position is too close to the sub and I lose all the lowest octaves. I was thinking of adding a second sub on the opposite side wall, then moving them closer to the corner behind the main speakers until it sounds fuller. Right now for music the sub is tight and musical but lacks bottom end ooomph. However, for HT I'm happy with it.