The MA7000 is far more linear sounding than the MA6900. The MA6900 carries a distinctly warmer aural signature. I'd take the MA7000, but that's just my bias/preference. Having never heard your GX-300's, I have no idea which amp would make for a more suitable match.
As for a DAC.. I suppose that depends on how much cash you have to spend. If you have a copious amount of change, I found that AMR's latest DAC is pretty damned nice. If you want something a bit more budget oriented, Rega's new DAC isn't too shabby (if you like that warm and musical sound). Both should form a decent match with the Mac gear.