Seems like my issue is the same. When I removed xlr's from the Krell, the ground loop appeared only when the right channel xlr was plugged in, absent the left channel cable. Trading the right xlr cables still induced some hum although less from the left channel with the right channel cable. So, it appears that it is induced by the right channel cable or there is a ground fault in the right channel of the amplifier. This unit has tripped ground fault breakers too and I suspect it may have a fault internally on that channel if the cable is not to blame. Will check with another xlr cable. It is curious why the NAD didn't haver the problem, but it doesn't appear to be the fault of the D90 in any case. It appears that the right channle is susceptible to induced hum from input cable susceptibility to stray power fields. I will try shielded cables.
Also like to add that this is a really good product. It definitely meets and probably exceeds the NAD M51 for SQ and feels like an upgrade with HQPlayer providing DSD 128x with ASDM7EC and Sinc-M in DSD direct. It is so clean and I am really happy with it so far.