Hi Dan and Anthony,
I only use REAPER so can't ascertain if this is happening elsewhere.
You will need keen ears & a quiet room (or just a spectrum analyser) to pick up on this sound.
In the first noticed instance of the problem, there were distortion plugins downstream which helped to make it louder. In those cases, the only audible sound was the tone, whereas the second time I noticed it, the plugin must have been passing the original audio since I didn't notice that the track in question was 'muted'/not playing.
In both cases, I disabled all modules, but had enabled transformer output and added a bit of output gain (~6dB). As I mentioned earlier, disabling the trafo didn't make the 12kHz tone disappear.
I'm in the middle of a bunch of demanding projects, but will get you some new information as soon as I can.
All the best,
Dax.
