I've had the same issue with my M80s; I've corrected it to a point where I don't think there is much else I can do. The system's low end sounds better but still not ideal (for me that is). From my listening position, if sit on the couch, it sounds like I'm into that bass null; if I stand up, I can hear a big difference in the low end. Weird.
In any case, what I had to do to improve the bass response was to move the speakers: distance from the wall, distance between the speakers and the toe-in. I played with this countless hours, but in the end it was worth it as I've some improvement for sure.
Now that you've read all this... you'll probably find it useless as I don't think it applies to your case?
In an effort to at least give you something useful, I would try a different set of interconnects (and I would've suggested speaker cables as well if not in-wall/cabinet speakers) and try a power amp.
One last thing: if you have your receiver plugged into a power bar/conditioner/surge protector, try switching it directly into a wall outlet; the reason I'm suggesting this is that I've heard with my own ears, where a power conditioner was taking the edge off an amp at louder volumes.
Last edited by AdrianD; 09/28/12 11:44 AM.
