[LAU] Ground lifts, TRS, etc. to PA? USB/other signal crossover to audio?

[Victor]

I'm not sure if I have understood your problem correctly. You refer 
first to
>   really odd high-pitched non-static signals which are contaminating 
> the audio
and later to
> back then, it was a 60-cycle hum
My experience is with a high-pitched signal of stable frequency 
(actually mine was a hiss of 8000khz dropping to 6000khz for about 
second at the end). It seemed to appear "randomly", and was very annoying.
I first checked for correlations in my apartment (does the signal appear 
in sync with the engine of the fridge, and this kind of things). But the 
problem turned out to be outside: the elevator! Although my apartment 
has no direct contact with its cage, everytime someone would go up or 
down, it would trigger that hiss. Once I found that, it became 100% 
reproducible.
Searching in that direction I found that other people experienced 
similar problems, especially in recent flats with recent elevators (mine 
is 2 years old). The problem is linked to a kind of transformer which 
softens the speed variations of the elevator. The device is known to 
cause interferences and in theory it should be properly insulated... But 
those who install it sometimes skip that.

Everything I tried to get rid of the hiss failed ... apart from 
disconnecting my loudspeakers from the soundcard. That would work, but 
it was not very practical. I suppose the metal coils in the speakers 
acted like a kind of antenna for the interference.
So for a while I just notched out 8000khz and 6000khz where the hiss 
occurred during recordings. As it had very narrow bandwidth, I could aim 
the filter right at it and not loose much of the rest.
Then two months ago I upgraded my speakers, and no more hiss :-)

I'm not sure if I have understood your problem correctly. But if it's 
high-pitched and seems to appear randomly, you could check for something 
similar.

Good luck,
Victor