Sometimes, troubleshooting is a pain. I had intermittent bursts of static in one channel. In the process of troubleshooting, I found one bad tube socket (replaced) and a bad XLR plug on an interconnect (got that re-terminated by manufacturer at no cost). I retubed everything -whole lotta tubes, almost none new stock. Still, despite this, and before sending equipment out for repair, I did a cross-channel exchange of the battery packs on my line stage- a fairly sophisticated device. In the process, I saw a bit of "fluff" on the metal contact plate. Dusted that and noise went away.
Moral: It could be something really stupid. I don’t know if there is a tech who could test some basic stuff at your home- like a house call. I almost resorted to that here in a town that’s thin on hi-fi (but lots of tube amp repair people in Austin for tube guitar/organ/other musical instrument amps).
I would also ask Grypon by phone (though you may have to set it up by email first) to tell you where they think the problem might lie if it is indeed the problem. Sometimes, those discussions give you leads to check something else.
Sympathize with you in suffering that "what’s the problem" phase until you suss it out. Some of the logic of troubleshooting is straightforward in terms of switching channels, trying to isolate to a particular component. It’s almost always got an answer, though. If that isn’t your thing maybe, depending on where you are located, somebody qualified could help do the troubleshooting. IT can be a PITA>
Good luck, you’ll find it and fix it.