A second subwoofer would help even out the bass response across the room. If you listen while moving around, or just not in the sweet spot all the time, I think that would provide a significant improvement. Probably more so than any amp or DAC would.
Although, adding a miniDSP 2x4HD in addition to a 2nd sub would also provide you with the ability to tailor your response to your personal taste.
5.1: Power - Denon 4308ci, Fronts - Def Tech BP9040's, Center - Def Tech CLR 2000, Surrounds - Def Tech PM1000's, Sub's - Rythmik E15HP x2, Display - Panasonic TC-55AS530U, TT - Rega Planar 2 w/ Dynavector 10x5