Need help from Pass X-600 + ARC Ref 2 MK1 user


I am matching Pass X-600 with ARC Ref 2 (MK1) and encountering a strange hum issue. I have tried couple combinations and it seems like the hum is coming from only X-600 + Ref 2. X-600 and Ref 2 do not seem to have any problem individually. Following are the combinations that I have tried and they show no hum at all.

1. X-600 + Pass Aleph P
2. X-600 + Wadia 861 (using build-in preamp)
3. Mark Levison 331 + Ref 2

I have also tried couple power conditioners and even power regenerator, but the hum still. I was told the hum might be from power & preamp impedance mismatched. However, it shows pretty healthy from the specifications. Any comments on this will be appreciated. Thanks in advance!
jshaw1004
Sounds like an internal ground-loop. I fix lots of these in my mods. They are common even in high-end gear.
Internal ground loop? If that's the case, which unit needs to be fixed? Power amp or preamp? Thanks.
After reducing the output gain from preamp, the humm goes away! Thanks for all your comments!