I'm not sure how much this will help, but when I compared my EP500 to the PB13 Ultra, the difference was minimal until I played with the phase settings. Both subs were next to each other in the best spot in the room, set to 0 phase. I don't know why(well, the room I suppose), but the sound improvement was dramatic on the PB13 when the phase was changed to 180 degrees, a difference I couldn't replicate on the EP500. I don't know if phase experimentation would be worth a try in your case with 2 subs, but worth consideration perhaps?
