Amplifier Hum

My 5000 has recently been exhibiting a low hum that is especially noticeable when the system is powered off and the amplifier is in standby mode. The amp is connected to a Model 976 and power is triggered from the pre.

The hum sounds like a 60-cycle hum but the power cord does not include a ground pin, so attempting to isolate the ground is not possible.

Thoughts? Anyone else seen this?

I had that problem with my 7500 and it was caused by dirty AC power.
I thought it was a ground loop problem so I tried the cheater plug but no difference.
I bought a Channel Islands AC filter and all of the problems went away.
Not sure if they still make them but Emotiva makes the CMX-2 that should do the same thing.

I will check those out. Thanks!