Right now I think my setup sounds pretty good.
My advice - if it ain't broke don't fix it chief. Spend the money on music.
Most of the dacs will be a sideways change at best. The idea about a sub is an excellent one. Don't know your listening room so I cannot recommend size. I just went through a sub purchase exercise and picked the Paradigm sub 12 after listening to the Rel B series and the JL. I wouldn't even consider the B and W sub.
OK, I've listened to the Benchmark and the DACMagic in very good systems but not the same systems and not head-to-head. Still, I did not hear, from either unit, the kind of highs that I would want to live with for a long time.
DACs I have heard which did give me the smooth, clear highs I can enjoy include the Ensemble Dichrono HiDAC, the Audiomat Maestro, the Stellavox ST-2 and the Apogee Mini-DAC. Of these, the Apogee is the cheapest.