Cannot record

[PGA]

Just a thought...
Much of the investigation so far seems to have focussed on the possibility of it being some kind of "ground loop" feedback. But doesn't that usually manifest itself as a hum close to mains frequency (60Hz in US, 50Hz in UK)? We know that the makers of our PCs and sound cards are trying to stop us making "illegal" recordings. Could it be that the "high-pitch hum" is actually a design feature of the soundcard to make "Stereo Mix" recordings unusable?