Yup. This trick worked for me too.
I popped out the power switch, rotated it a few turns, and pushed it back in. After moving the twisted red wires away from the circuit board, SHAZAM! No more buzzing from the left channel.
This amp is dead silent. I can turn it up 100% even in non-OTL mode and there is no noise at all. Fantastic! Next, I'll probably upgrade the output caps to Auricaps or something similar.
Hmmmm... I wonder if a constant current cascode source could be done on the MG Head? It did wonders for my Bottlehead Foreplay. A CCCS mod may close the gap with the Melos.
(now researching... heh heh heh)