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.
This happens on the cd input,

Hum through both amps? A 60 cycle ground loop hum?

What happens if the CDP ICs are disconnected from the preamp RCA inputs?

Just the preamp connected to the amps. Hum? No hum?
I will let you know. I tried a different preamp on same set up and no hum.
I had the same problem when I connected my CJ preamp to my Ayre V-5 with a SE cable and left the Ayre in balanced mode. It stopped when I put AQ interconnects between the 2. With every other cable I get the hum.
I also have similar ground loop hum problem. I fixed it by need disconnecting the earth from power plug that connects to CD player.
Had it once, was the interconnect.