Drew, a couple of comments that might not solve the mystery, but should be relevant: my understanding is that in the Bose system neither the cube surrounds nor the small center can handle below about 250Hz and that the bass module has a crossover to take care of even the upper bass frequencies, so you're not actually hearing the "itty bitty" speakers there. When you speak of an 80Hz crossover on the EP500, if you mean that the control on the back is set to that, then you should set it to "Bypass" instead, in case this is causing some mid-bass weakness in the "small" speakers. From what you say, you've apparently already experimented with various crossover frequencies on the 2600 in the 80-120Hz area.
Your overall impression of the VP150 doesn't seem to be that of other users, so you might contact Axiom on the possibility that it's defective. One suggestion would be to temporarily move one of your M2s into position as a vertical center(replacing the VP150)to investigate the likelihood of a problem with the VP150.
