I also have similar ground loop hum problem. I fixed it by need disconnecting the earth from power plug that connects to CD player.
Hum in Audio Research
I have been using the AR REF2 Mark2 in balanced mode with no problems. My nephew wants to buy it, but it hums with his VTL 450 monos using single ended cable. Has anyone else run into problems with AR preamps run single ended? The hum increases with volume. This happens on the cd input, tried cheater plugs with no success.