I've tested another MP-1 v2 which has a 2KΩ T3 Trimpot. It was initially adjusted about 1 o'clock and was 0.87KΩ. The reading from U8 pin 1 was 11.2Vp-p and adjusting T3 "SEEMED" to have no effect, just like mine. Upon further examination, if you are not looking directly at the oscilloscope while changing T3 you won't notice a change. This is a very subtle adjustment and it barely moves the wave. Until I was looking at the scope and sweeping T3 from stop to stop, I didn't notice any change.
So I went back to mine swept it back and forth and then setting it to maximize the wave and bringing it back a "hair". Took a measurement of T3 and it was 0.88KΩ and around 1 o'clock as the other MP-1.
I'm calling the S.S. setting solved.
MarshallJMP, I know you've been through a lot of these and can you confirm that the adjustment of T3 is very subtle, and hard to see unless you sweep the trim pot?