Sorry, must be a mistake, I've never actually owned a Mesa, and I've never solved the parallel loop problem without having it modified to series (I had this done to a small Laney amp) . My local tech guy showed me on the schematic what he did…it was simply a matter of removing one wire and moving another…something like that. It might be slightly different on the Mesa, but I don't think the mix knob will even come into it…with series you won't need it. Best thing I think is to take the schematic to someone local  who knows about amps.