This maybe more simple than you are thinking, but the newer Denon receivers include Airplay. You could get an iPad mini - which should hold 50k songs - (and possibly a newer Denon AVR), and simply stream music from it to both zones using the DeRemote app. Solves both the client and GUI issues. I'm typically NOT an Apple fan, but I think AirPlay is a decent feature. Now if the music is saved in a truly lossless format.........