@atmasphere Ralph, thanks for the discussion on this. Helpful, educational and appreciated.
The only way I've found to get around that is to avoid that sort of topology- either go entirely single-ended and get that 2nd harmonic or go entirely fully differential and get a 3rd harmonic (although at about 1/10th the level that the 2nd shows up in a single-ended circuit). Since the ear is relatively insensitive to either other than contributing to 'warmth', 'body' and the like, this seems to be the way to go since you can't put enough feedback on a tube amp to really tame the distortion properly (or most solid state amps for that matter)- you wind up just making it brighter and harsher.