I have a hard time believing that the ground loop I hear in my headphones is also in my system. My system is pretty darn quiet “on” without music, but plug in headphones, and that’s a different story.
I’ve asked PSA, and got a ground loop troubleshooting guide. Wouldn’t it be present in all output, and not just cans?
Have you tried a different headphone cable?
It is dependent on the sensitivity of your headphones vs. your amp/speakers. Headphones frequently have much higher sensitivity but sometimes with for example planars they may not. Try plugging in only the headphones (no pre outs or inputs) and compare the difference. Then start adding inputs and then finally the outputs to find the source.
Did this just start happening? Or is this the first time you’ve tried headphones with this Pre?
Thanks for the ideas.
It’s an unsolved mystery with multiple cans and headphone cables. I know It has been mentioned that the BHK is good with headphones by the big man himself, so I’m doing it wrong.
My speakers have low sensitivity and the cans I’ve used are higher sensitivity.
Speakers: 86dB/1W/1m
Cans 1: 111 dB/V @ 1 kHz
Cans 2: * SPL 1mW: 99.8 dB
One cable choice is a bit long.
I have a friend about to loan me some Hifiman planar cans, so the search for goodness continues today. I’d like not to need a crazy expensive amp for a good headphone set-up.
Multiple attempts over the years. Never liked what I got out.
I ordered a BHK pre with the October sale. Worked fine with out headphones. Decided to try the headphones and experienced the same thing. This was with in the 30 day trial period. Called and tried a few fixes over the phone and no avail. They then gave me an RMA.and replaced it. The second one is with out headphone problems.
I just tested my BHK Pre. No issues.
I tried the Hifiman Arya’s, which need more than my Grados and B&Ws. They also presented the ground loop hum. That’s another separate cable combo and cans.
Here was the big test: unplugged everything, still have a ground loop out of multiple headphone configurations. Still under warranty! Looks like I’ll be driving up to bug old Paul and finally hear the FR-30s and BHK-600s. 
That is the only test that really matters. Nothing attached but the headphones is telling.
It’s a PIA is what it is, but a necessary step.
Are we discussing divorce or your Pre? 
I’m able to discuss both in a “got my cake and ate it too” sort of way, but think cake only relates to the divorce. I best stop there…heh