what is this circuit?

[cugel]

here are some gut shots of an amp i bought used (built by a small builder out of a garage type thing, not a major booteek builder BTW) that i love and has some interesting features. for the most part this is a standard 18watt marshall thing with two channels: an EF86 with hi-lo inputs and a vari-tone switch and a push-pull volume pot. the other channel is a typical TMB 18watt channel with a MV. There is also an effects loop. So, engaging the push-pull on the ef86 channel cascades it into the other channel. I was trying to figure out how/where in the circuit since it bypasses the TMB tonestack but not the TMB volume or MV. after looking at the guts i have am confused. What are those diodes doing going to the MV pot? What is the pot doing between the ef86 and the first 12ax7. i know this is the cascade circuit but why the pot??

hopefully some bored amp gurus are feeling talkative today
thx