d3 - Welcome to the circle! Let me jump in...
Some of the advice given here is just good "general" advise but I think none will solve your hum problem specific to your Tempest situation! And no isolation transformer will help here either.
And BTW, Folsom hit the nail on the head!
What you have here is NOT a ground loop but a luck of GROUND! Let me explain...the RCA's used on the tempest are PCB mount devices fixtures that are mounted (soldered) to the main circuit board. What has happened in some instances, and I think is your problem, is that the "ground pin" common to both channels in the output RCA's is broken (open) - and when you mentioned in your first post that connecting one channel from the preamp to the amp was no hum, but when connecting the second RCA (channel) created the hum - tells me that with only one plug connected - the "broken" pin was barely making contact (no hum) but with the second RCA inserted created enough physical pressure to pull the ground pin (open ground), creating the hum on both channels!
The only solution is to change the RCA's outputs...I suggest you contact Klaus and send your unit back to correct the problem.
For the record, this issue has happened only a few occasions in the past mainly during (hard) shipping (heavy box drops) and also by using heavy "locking" interconnects - forgetting to unscrew the fixture when plugging/unplugging and wiggling the sh..! of the plug and RCA - braking one (ground) or all pins connected to the circuit board.