I have also weighing the options between the EP350 and the EP500. Recently I came across an article about the Emotiva Ultra 12 Sub.
http://emotiva.com/ultra_sub12.shtm

Audioholics gave it a rave review.
http://www.audioholics.com/reviews/speakers/subwoofers/ultra-12

It's on sale for $429 right now.

Listener - Why would you go with HSU or SVS instead of the EP500?

-Mike