Krell KAS amplifier hum

All was unplugged from the back of the pre, I still have "hum".

I ran a ground wire from the pre power cord (plugged into the back of the power distributor) to the ground of wall outlet the power distributor is plugged into; the hum remains...??

Many thanks for all advice given, 
Jose

@jea48 is the best there is for troubleshooting hum. You need to follow his instructions to the letter. Remove the power conditioner from the system. You should only have the monoblocks and the preamp hooked up and plugged into wall receptacles.

jea48 said:

You can also use a 2 wire IEC power cord if you have one instead of using your existing preamp power cord. You won’t need a ground cheater then.

That won’t work..... The male ground pin on the IEC inlet on preamp would be in the way.

jg2077 OP41 posts

12-11-2019 7:52pm

All was unplugged from the back of the pre, I still have "hum".

I ran a ground wire from the pre power cord (plugged into the back of the power distributor) to the ground of wall outlet the power distributor is plugged into; the hum remains...??

Many thanks for all advice given,

That won’t work.  Adding a parallel ground path will solve nothing.

Use a ground cheater on the ARC preamp power cord plug. (Nothing connected to the inputs of the preamp. Again just the preamp and the 2 amps. Nothing else)
You need to lift, float, the equipment ground from the chassis of the ARC preamp. That will break the ground loop circuit.

By chance do the Krell amps have a ground lift switch on the rear panel? If so the switch will lift the signal ground of the amps from the safety equipment grounded chassis.
Just a note. At this point I am hoping the problem is the equipment ground of the 15 amp convenience outlet circuit. The ground cheater on the ARC preamp power cord will tell us that for sure if it breaks the ground loop and hum.

Take both mono block out of the circuits, connect a headphone amp to the ARC ref3 output and listen if hum still exist?

In some case, hum is caused by upstream equipment.