Krell KAS amplifier hum

Rule No1: Only put these sort of amps direct into the wall.

Those Krells of yours have an incredibly "stiff" (low esr, low impedance) power supply and great output current delivery that’s why they can drive anything even the worst speaker load yet still get great controlled bass, this is only got by plugging direct into the mains.
To put any power conditioner between the Krells and the wall outlet and you introduce series resistance which lower the power supplies stiffness and that bass and ability to deliver huge current is diminished.

Cheers George

Is there a "conditioner" I can use to minimize/eliminate the mechanical hum

Beautiful amps BTW.

There is no conditioner I know of that can take idle current plus music playing current into MBLS’s of a pair of KAS Krell monoblocks and not be near it’s limit.

If your hearing hum through the speakers.
KAS Monoblocks can give hum by their very nature of being mono blocks, they should BOTH be plugged into the same heavy duty mains outlet to stop any earth loops through the house wiring.

Also use some "earth cheater plugs" on the preamp and source/s so they get the earth from the Krells through the interconnects, this way you stop any more earth loops through the house wiring.

Cheers George