The quality of the sound of my 2 EP-400s for 'music only' is simply outstanding; however, I've never used them for the high demands of HT. Four undoubtedly will have enough horsepower for the rigors of HT as was demonstrated here in 2007:

http://www.audioholics.com/subwoofer-reviews/axiom-audio-ep400-subwoofer

The EP-400 has been significantly tweeked since this article in 2007. Mine are the latest version that continue to impress me with their exemplary performance. Get your out wallet if you purchase four though, ha!

Personally, I would go for 2 X EP500s for HT - way cheaper too...

TAM