A post-phase inverter master volume had already been installed in this amp, and it had ben modified slightly in other ways as well. I put it mostly back to stock, and converted the master volume to a phase inverter bias adjustment control. It doesn't bring the volume all the way to zero, but it sounds better than any other PPIMV I've tried in a Twin so far.
I also replaced some worn out components and all the electrolytic capacitors.
