Original Poster, ShadyJ has been giving you solid advice here. Every sub you've mentioned will be a downgrade from a Epik Empire. Unless you go DIY- there isn't a sub that will sound better new for $600. (assuming your epik empire works properly). You need to take some advice here from several of the guys and move the sub around the room until you find a spot that suits you better - OR relay your equipment setup, receiver, sub settings, movies you aren't happy with etc and let us help you with a better understanding of the problem and your expectations. Just buying new subs willy nilly is going to lead to disappointment in this case. The Epik Empire is a very fine sub, and is typically compared with other $1000 subs like the HSU VTF-15H, or the SVS PB12+, etc - - - not $600 or less subs.
With this exception.
If you live close to Flordia, there is a guy selling a pair of older passive SVS 16-46 for $600. Those subs will kick the pants off the epik empire in the lower bass regions, and keep pace in the mid and upper bass regions.