[Bluesbreaker]

Also: Make sure there is no feed back loop: for example, main outs to
a cassette deck, whose outputs then come back to inputs on the mixer.
This has happened to me with Mackies, and it weirdly affects the
output levels, even if the cassette deck is potted all the way down!

I've eliminated as many variables as possible. Single mic into any
channel. Monitoring with headphones. If I C-R/SOURCE the main mix, I
can see (and hear) the panning problem. If I route to the ALT-3/4 all
is good.