Add a subwoofer or not for two channel only?

Bi-amping won't get you extra bass if your speakers can't produce the low frequencies. If MORE bass is what your after, you should concider a) new, full range speakers b) add a sub or c) move the speakers a little closer to the corners of the room which might help.

All have their draw backs. First, if you get new speakers, you may then need a new amp to push them. Second, I personally would not want to run speaker wire from the amp to the sub to the speakers (I'm assuming you are using an analoge pre that does not have a LFE out for subs). If you move the speakers closer to the corners, you may get unwanted reflections and will have to treat the room to control them. The third option is the cheapest and easiest to do and is where I would start.

REL Strata III, either one or two. A small sealed-box sub that will match very well with a small room, adding deep bass without going so deep that it overpowers the room. These are very good subs and work well in either mono or stereo setup.

Hello,

I have always found the old Janis subs very musical with the Janis Interphase 1a amp. I took the first responders advice and bought some nice full range speakers and just sold my Janis. They can be had cheap these days. I always found other subs too boomy and they did not blend well. The REL subs are nice also.

Get a sub, friend!

I'm currently using a Servo-15(-3db at 17hz) to augment my B&W CDM-9NT speakers(-3db at 38hz) in a room slightly larger than yours.
Surprizingly, the Subwoofer actually has slightly less apparent bass output than the speakers alone, and there is virtually zero difference in bass "impact" and "slam" with most music.

I believe this is due to a large "suckout" at 50-60hz in my room. The ideal solution would be to remove the suckout and accompanying peaks using large bass traps.
Since your room is dedicated to music, the bass traps should be much easier to implement than in a typical living room, and may even mitigate the need for a subwoofer.

Build a Better Basstrap

Check out the Janis 3a/1a combo just posted for $250 smackers in Atlanta. That rocks!!

I have the Piega C-3 and run them with a Rel Strata 111 and tried several different settings on the Sub, I finally have gotten seemless coherent music. Very satisfied.

I too have a small room with speakers that go down to 35hz. I recently added a Rel Storm sub (the dealer told me that the Storm has a richer bass while the Strata is more taut.) In any case, I was pleased with the significant improvement in not only my now seamless bass response, but the improved soundstaging and air. The Rel system of not using a crossover, but instead passing the signal to the sub from the amps while allowing the speakers to play full range is ingenious. The bass and midbass seemed enriched rather than added on to (I can not tell where the sub.) The maker of my custom amps was recently listening to my system and gushed on about the bass and particularly midbass sound of the system. He did not know I added a sub and when I told him he was flabbergasted because he said previously he could usually tell when a sub was in a system and hated how it usually detracted from 2ch listening. It was much, much easier to set up the Rel sub in my more difficult small 2ch room, than my dedicated ht room (I use different subs there, although I will probably change to the Rels after my current experience.)