Kalbi .. , don't know much about the Mac or SF impedance curves. So I only add this comment because it might help focus your discussion with MAC tomorrow.
As I've come to better understand with my own gear, using the lower impedance taps on my tube amp enables me to drive my speakers with a lower output impedance source. Of course, my amp uses output transformers and negative feedback, but doesn't have built-in autoformers like some MACs.
Frankly, I am a little puzzled why one would want to use higher impedance taps if the price is higher output impedance. Most tube-heads may say use whatever taps sound better to you. That's fair advice ... but what sounds better to you might be acoustically colored playback especially if the amp's output regulation varies more widely with respect speaker impedance which varies as a function of frequency.
I've killed this issue a zillion times already and if I start it up again, I'll start a negative feedback war. Yes ... NF comes with a cost. Read Ralph's white paper on the Voltage and Power Paradigms. But understand that a tube amp that uses NF does not perform like a pure Power Paradigm source. It's kinda' a hybrid which is good in some respects, but also inserts TIM and odd-ordered harmonic distortion into the output -- not so good. But life, and amplifier design, is about compromises. You takes the good with the bad.
Cheers,
Bruce
Perhaps Ralph or Al will edit my glarff so it makes more sense.
