I think it may be clear to some (most?) folks at this point, but I want to (hopefully) help others understand what sort of problem you have. Given the above statement this is NOT a ground loop problem, rather it is high frequency “hiss”. To audiophiles (and other music related folks) “hum” infers a ground loop which is manifested in a 60 Hz or 120 Hz “hum” from the speaker. Several posts talk about power, cheater plugs, etc. which are solutions for ground loops. This is an amp internal characteristic as Paul has stated.
Now, at the risk of annoying you, and that is NOT my intent, the noise issue when using single ended interconnects has been discussed previously. It’s not a secret and I would be surprised that PS Audio folks did not understand that. I wouldn’t expect you to know this since you indicate you don’t keep up with the forums, but I’m surprised (again) it was not pointed out quickly by other members. Sometimes we all miss the point …
Anyway, I would reiterate what has already been said that given the amount of money invested at this point a simple pair of Blue Jeans Cables XLR/balanced cables for less than $100 would be a cheap and effective way to solve the problem. Should you want better there’s Iconoclast BAV for less than $400. The point being you can get more than satisfactory performance for a small investment.
Good luck!