I've got both the EP500 and the PB13 Ultra at the moment. I also have your same setup minus the QS4's.
To answer your first question, there's no issue in choosing a different brand sub to go with your Axioms if it performs to your liking.
Since you'll be mainly using the system for movies, I'd definitely recommend the PB13. It's certainly faster, cleaner, deeper, with more "slam". Not by alot, mind you, but enough to make a difference. The EP500 is a wonderful sub, but the PB13 is in a different price category and is more comparable to the EP600, as jakewash mentioned.
Then again, if you've got a smallish room, the EP500 would give you nearly the same impact for much less cash, IMO. It's also about half the size!
