The third channel is missing gain and oomph (bass), channel 2+ has more on tap.
My first guess was that it has something to do with the preamp tubes but these are fairly new and it also happens with other tubes. Is there something more I can test?
Switching around the preamp tubes wasn't helping so I brought the Herbert to a tech I know. He solved the problem, with some help from Peter I believe.
The amp sounds fine and dandy again