Right now my sub is the older AX500a. This sub plays quite loud, and melds really well with my M3s which are serving as the mains in my HT setup. I plan on slowly upgrading the speaker array to a 7.1 setup with towers in the front and the M3s moved to the back (it's sad to see them relegated to the rear...they deserve better .

What kind of difference in bass can I expect with the EP350 vs. the AX500a? The EP350 is twice as powerful, but it's a much smaller speaker in terms of cabinet size. I guess this is a question for Ian, or somebody else who's familiar with the older speakers.

The AX500a is probably most similar to the EP175 from the new line. If you have the space and the esthetics would be OK, I would think about keeping the AX500a in service and adding an EP175 to the system. If this won't work for you then a single EP350 is the way to go for the upgrade.