I prefered the Harmonics setting on my old A´s. Seemed more balanced and really made the harmonics come alive. It had enough gain and sustain for me at this setting.
The Mid Gain is cool for leads, but added a somewhat artificial, slightly harsh fizz to the tone at low volumes, at at higher vol settings it would feedback like crazy with high gain and LD settings. It also seemed to bury the dynamics a bit.
It will be interesting to see how my new B version differs. Or if my taste has changed since I last owned a Mark IV. I´ve also bought a few new guitars since that time.