Just as a follow up, I didn’t get a chance to actually try the different connection options but found these comments in the MC2200 owners manual. Not sure if they apply to other Autoformer design McIntosh amps:
"If a load impedance is used that is lower than the output
impedance tap, then reduced power and possible distortion
will result. If a load impedance is used that is higher than
the output impedance tap, then neither the signal nor the
amplifier will be harmed but the power available is limited."
16 at 1 ohm output, 50 at 2 ohms output, 30 at 4 ohms output,
16 at 8 ohms output".
So it looks like the taps have different output impedance and the taps with higher damping factor should theoretically give a "tighter" bass, all things being equal. Assuming the same behavior between MC501 and the MC2200, this might explain why denman prefers the 2 ohm tap for the lows.