All that existed already, even without replacing the mixer, just read the first post by GordonC in this thread ...
Thanks! I read that previously, have learned a lot from this site and others, and very much appreciate everything everyone has contributed. My own mixer mod is a simple emitter follower for each oscillator, with a jumper to put it mostly back to stock response if desired (though I can't imagine why anyone would). I'll post the schematic and some pix soon.