Brian, welcome, but c'mon, give us another clue so that we can figure out where you're from!
As to the sub, I can strongly recommend the EP500 which I use if your budget would extend that high. However, although you say that price isn't a "major consideration", the fact that you've bought the modestly-priced, but excellent, M3s, indicates that something more in line with that price class might be more comfortable. The EP350 would be a good choice, or going outside of Axiom for a still lower cost, you might consider the fine Hsu STF-2.
Enjoy the fine sound through your M3s.
