The EP350 and EP500 have a lot of differences. They look the same because they share the same cabinet but the EP500 has almost double the amp power, a longer x-max woofer, and a way more sophisticated control of the frequency response and max output limiting. This means it plays a lot louder, a lot lower, and never gives you a hint of straining. I think you would find it a nice upgrade.
Thanks. I know what my next upgrade will probably be then, if I can get it for that price range (around $800). I've still got the original shipping box. I'm strapped for cash until November when I'll have my new TV paid off.