Best place to put a single subwoofer?

I know there are many opinions regarding subwoofer placement. I have had multiple subs (4) all in the same listening space. Way too much! I have had matched pairs hooked up in stereo. Right now I own only 1. It is part of my dedicated 2 channel music room and not a theater system. I have a pair of MBL 101e Mk2's which are excellent speakers IMHO, and they produce POWERFUL bass in their own right. However I am a bass freak (as noted above), so the harder it hits the better. Plus I tend to listen to a fair amount of rock music which I like to play loud, (hence MBL 101e's). Right now I have my sub positioned dead center between the two Radialstrahlers and it does ok. But I feel as though I'm missing something and that I ought to do something to increase performance, although I'm not exactally sure where to go with it. Not too many places I can move it to, but input and suggestions would be much appreciated. I have tried moving it closer, and then farther away from the wall but have not noticed any dramatic improvements. What about bass "suck out"? Could I be having some cancellation issues? What things would you Audiogoners do to improve on (already good) bass performance?