What size subwoofer to get?

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.

I had this dilemma once and decided it was far more cost effective, and would also yield overall better sound, to sell my speakers for a fullgrown full range model. With the seperate sub imaging will be lost. As I understand it you should crossover one octave above your speakers bottom extreme for best results, so around 70Hz, which can be done with the Outlaw Audio bass managment system (www.outlawaudio.com). Also worth note, of the thousands of songs I have on disc of all types of mucis, maybe a dozen have anything portrayed below 35hz, of those I have only 2 tracks with any info from 25hz on down, and it is electronica/techno/trip-hop type of music, so unless you listen to such musik, or I suppose organ music, you're not missing much....

Socrates .. I disagree with some of your suggestions:
1) If speakers are rated to 35Hz, and you cross below 30Hz (it is usually recommended to leave the main speakers unfiltered, and cross below their cutoff frequency ala REL, ACI) then this will have no effect at all on the stereo imaging ... such low frequencies are not directional.
2) Of the several hundred CDs I have at least 75% benefit from the subwoofer, whether it be added weight in the bass drum kick, the sense of the scale of the venue on live recordings, timpani on orchestral music ... I could go on. The type of music that DON'T benefit are extremely limited. Of course the effect is most pronounced with electronica and organ music, but it is very present in most other types of music.
3) Separating the source of sub-bass from the main speakers gives flexibility in placement to allow good imaging on the mains, and good room loading on the sub. Full range speakers don't give you this option.
4)I have heard good full range mains ... in a large room they work very well. I have heard a good subwoofer ... in most rooms it can work very well. Both options are possibilities. However you seem to rule out the subwoofer approach without having had any first hand experience with one. When you say you decided .. was this based on listening, or just a preconception ?