Sorry I can't provide anything specific, as I've never had a SS100 open in front of me, but I had a couple of thoughts....
I'm going from memory here, but if the layout is anything like the BE100 (and it probably is ) you may have a couple of options:
On my 2015 BE100, it looks like the relay that switches in the HBE triode ( or what would be the boost triode on the SS100 ) has an unused half that could be used to switch in a second master volume. If this is the case, then adding the second master would be a pretty simple process.
If, on the other hand, you HAVE to have a second gain control, as well as the second master, then you would be looking at a much more complex install that would require an additional relay and some more extensive circuit surgery.
On my BE100, switching in the extra triode provides the right amount of gain bump, but I could see where, if the amp was being used live, the small increase in volume is not enough for your solo's to cut through the mix. Adding only the second master would fix THAT issue.
I don't know that i would try to add the second gain pot, but it's up to you. A local tech could likely do the second master mod fairly easily, but if you are going to consider the full gain and volume mod, I would probably just send it to Dave.