Insane ground loop; anybody wanna try?

If you are using inter-connects with arrows on them , the arrows must point to the Pre-Amp .......Most of the time people think they should point to the way the signal goes, which is wrong........The arrow points to the grounded end of the cable (which is only grounded on one end)  This will cause this type of problem..........autospec

Consider trying an Equi=Core PC that is actually a Balanced Power device using a proprietary transformer to split the 120V into 2 out of phase 60V halves, and when they are recombined the non zero summed noise is drained to ground, lowering the noise floor dramatically. Tim Stinson of Luminous Audio couldn't get rid of a ground loop hum at the 2016 Capital Audiofest that was preventing Tim from using his Arion Phono Stage in a system, Sunil of CARE Audio loaned him an Equi=Core 300 and solved the hum instantly.It's worth giving Mark a call at Core Power Technologies.

In my best system I run AG horns, my current Trios are something like 102 db sensitivity or even higher.  They pick up tiny hums, noise, sometimes I think even insects walking around stealthily in my basement. I've run many amps, low and high power tubes and low and high power solid state, and always encounter what I call ground loop issues.  Get out your cheaters and lift all the grounded power plugs, disconnect unused circuits throughout the house, etc, to no avail.  Still the damn hum and noise.
My cure, courtesy of Steve McCormack, now running SMc Audio.
The music source is not the problem.  Make a triangle from your pre-amp to both mono blocks, all three legs.  Ground them to each other, all three legs. Unhook your sub if need be.  Voila, hum is gone, as is any stray noise.  Now, you would think the third leg is redundant.  ie the Pre separately linked to each block, and thus the blocks have a hardwire ground to each other via the Pre.  Well, I did that, two legs instead of three,rid myself of most hum and noise, yet still had some audible hum and noise.  Connecting all three legs equals dead quiet, and perfect source music.
Sound illogical?  Too much work? Hey, it blueprints perfectly in the ground plane, and I use solid copper wire, like 14 or 16 gauge, doesn't need insulation.  Simple really. Avoid 12 gauge solid, not malleable enough, and any stranded copper, not worth the fuss for grounding.  I don't know how you feed music to your sub, but I'm sure you can hook in after you know the noise is gone.  Good luck.  Best regards, Joel

I've been busy but have read through everything each of you contributed. I'm very grateful to everybody for taking the time to offer thoughts. Thank you.

Here's the update:

I used what time I had to reroute and reconfigure the two electrical circuits in the house per my last posting. My back is sore, my fingertips roughed up. And when I finally had everything put back together... Can you wait?! ... No change. I kinda expected that, being a pessimist at heart, but was still disappointed. Hey, at least I lowered the ground resistance to near zero in the system's circuit.

So I kept poking and think I have probably found the real culprit: the Rowland preamp.

I should have tried some of these things first, as they're simpler, but alas, we all learn by experience, yes?

Turns out that when I run my DAC straight into the amps, the hum is barely audible. (It's a Mac server, so I can control the volume through iTunes.) That also seems to eliminate that digital equipment as the cause per se.

But here's the salient clue, something I hadn't tried before: If I disconnect all source cables from the preamp, and then disconnect the preamp from the AC power, leaving the preamp and amp connected ... the hum remains. Yikes.

I opened the preamp and looked at the power board. There is a cap with one leg connected to the ground screw via a PCB trace, but the solder connection is hair-thin. I've contacted Rowland to ask if that trace is supposed to be stronger, which I'm capable of addressing myself. But if that's not it, I'm awaiting their response, which I assume will include sending the preamp to them.

The greatly reduced hum has also allowed me to identify a little RFI in one channel, but I'll take one thing at a time.

If this stimulates any thoughts, I'd be glad to read them. Otherwise, this feels more solid to me and I hope it's the beginning of the solution.

Thanks again, all.

pbraverman, thanks for the update!