I think you can find a higher value for your subwoofer. While the EP350 is a good sub, I think you can do better for ther price (the 500 is a different story though). I say this based on what others have said. I have not personally used the product, but you may find a better value from something like the Hsu VTF-3, Outlaw LFM-1, or Hsu PB-12 ISD-V. While the first and last options I listed are more than the Axiom, they offer lower extention and higher output, and still come in under the price of the EP500.
As far as the upgrades go, have you talked to Axiom directly? They can give you solid advice that will help you choose the best system for your needs. Give them a call or e-mail.