Dana, since I haven't heard either sub, EP350 or EP500, I'll let others who have listened to both compare the differences. However, reg the actual upgrading...have you considered the idea of getting a *second* EP350 and running two subs. This may/may not work within your budget, but may improve your system over a single EP500, or perhaps you could go through the trade up program if you want to go with the EP500.
The DIY route is certainly worth considering. There are several kits out there that have already been figured out. Basically you just have to assemble them. I believe most of the subs on
ED Audio are available in kit or finished form.
Correction: Elemental has a couple of their subs available in kit form.
