The 2*a-b difference tone is called the "cubic difference tone" in most
works I've read.

I hear slow beats, warbles, etc. in just chords and dyads on my PC, Mac,
and synth and I ascribe them to slight mistunings. A cent or so
mistuning of the fundamentals becomes wider between the overtones and
noticeable.
If strong combination tones occur, the same kind of error enlargement
will take place.

--John