MarkT:
It would actually be better to use 100k for that reason, although
high impedances have issues with pickup and noise. The existing 50k seems like a not unreasonable
compromise.A better approach overall is to buffer pot wiper voltage with a unity gain buffer, then the summing
junction resistors can be a nice low (ie low noise) 1k or whatever. For a low noise microphone preamp
you'd have to pay close attention to such issues, but that's not the case here of course.
Thanks for this recap.
In fact there are 3 inputs and one of those inputs is actually coming from a mic x10 preamp (transistor based). So I don't think it should not make a difference.