Two subs in a small room....

Unless you want a real lot of bass.A lot of rock? Shaking the room with movies? Or if you want to catch the lower bass your
present speakers won't go down to,then go with one good 8" sub.With one good 8" sub,it may blend in well,and still give
you those lows you just want to hear.It all depends on what you want to do.

It's probably useful to think of "woofers" rather than subwoofers. Some subs aren't really subs at all, in the literal sense. (See the thinline models from NHT with response to 38hz). Remember that bass issues are pretty severe below 150hz in most rooms. Good monitors, together with a pair of (so called sub) woofers placed flush against the walls, will probably help mitigate these issues - versus floorstanders which place their woofers out away from the walls. Note that the bass response in such a monitor/"sub" system will be smoother, even if it isn't particularly deep.

Good Luck

Marty

10x10 room will tend to amplify 55Hz (1125ft/s / 20) - not ideal since many speakers have port tuned to about 30Hz and get already hump on 60Hz. Bipolar speakers cancel this effect by directing sound in both directions (canceling). Similar effect can be perhaps achieved by using two subs facing opposite directions. AFAIK sound at low frequencies is not directional. Two subs are often used to reduce room effects (peaks and valleys) and not for the amount of bass or imaging.

If you are looking to enhance lower frequencies without much care of accuracy, then by all means get a sub. Tune it low, and you will be fine. Reduce the volume if needed. You can pace a sub almost anywhere, for any reason and get good results...IF you do not demand accuracy and perfect synergy with the speakers, do it. I would only purchase 1 sub, 10 inches min diameter. Later add an additional one IF your space increases.

Look for a passive sub. RA Labs (Roy Allison) made some good ones and they show up in the used market at very low prices. Essential they pull out the lows and create a great mid range.