what i would suggest is that you make careful Fr. resp. measurements of your M80s at different positions until you get good (flat) freq. response in the lowest frequencies;
when you move the speakers, try not to move them more than 3 inches at a time, and make logarithmic graphs of the results.
try 3 inches away from the front wall (that will be an effective distance of 19.5 ins. from the speaker's front to the front wall), and for the distance from the side wall, make sure it's not a multiple of the other distances (woofer to floor / woofer to front wall).
always measure distances from the center front of the speaker box.
when you have found the best position, then see if the bass satisfies you. if not, then choose a sub that will supplement the missing frequencies, at least down to 20 Hz.
i would be surprised if you can find a sub that will give you -3dB at 20 Hz (with sufficient SPL capability) for $350.
you also have to look for a sub that sound "musical", and not like a boom box.
hope this helps. :-)
