here's a thread about the mod.
http://forum.grailtone.com/viewtopic.php?p=82589&sid=03e942ff9ffbff9d0af59c13f70f6961
In particular a comment made by someone who had the mod done to his amp.
PJRAKE said:
"the master volume still works and i can control the FX loop with the footswitch, however the...