I'm thinking that if you did not have the problem before rebuilding HV that something is wrong with supply voltages or you've put a connector on wrong somewhere. Always a good idea to check the voltages before hooking everything back up.
I'd start by disconnecting the aux board and checking the incoming supply voltages to the connector. You might have more damage than just the transistor, and you don't want to repair or replace that board and have the same thing happen.
The incoming +/- 12VAC is rectified on this board and used to supply +/- VS to the audio amp transistors at the lower left and right (the ones on the big heat sinks)