Brian, this is anecdotal as I haven't auditioned any of the subs in question (I have an older-design Axiom 350 that really is nothing like the current 350). Maybe it's not worth much, but here goes.
Everything I've heard about the 400 tells me it's a GREAT sub in a small room or with music. But since your room is relatively big, my gut feeling is that the 400 won't offer ANYWHERE near the performance as the 500 in your setup.
Although there are other makes that are generally considered a good value, Axiom offers free shipping (even to HI, I believe) and offers the advantage of matching the finish on your M3s.
FWIW.
And welcome, by the way!
