Lots of excellent and informative responses above, except that I believe some of them have been made without the realization that the OP’s monoblock amps are solid state. And the multiple output taps are created via autoformers, the result being that the taps have higher damping factors and lower output impedances than pretty much all tube amps.
Specifically, the MC601 has a specified damping factor of "greater than 40," which if accurate would at least in theory mean an output impedance from the 8 ohm tap of <0.2 ohms, and <0.1 ohms from the 4 ohm tap, and <0.05 ohms from the 2 ohm tap. All of those numbers will result in the amp essentially behaving as a voltage source with this and most other speakers.
So I would expect in this case that interactions between amplifier output impedance and variations of speaker impedance over the frequency range are very minimal contributors to the sonic differences the OP hears in going from one tap to another. The major contributors would seem likely to involve how the sonics of the amp itself (and perhaps also the amp’s maximum power capability) are affected by differences in loading of its output stage that depend on which tap is selected.
Regards,
-- Al
P.S: Bruce (Bifwynne), great to see you posting again!
Specifically, the MC601 has a specified damping factor of "greater than 40," which if accurate would at least in theory mean an output impedance from the 8 ohm tap of <0.2 ohms, and <0.1 ohms from the 4 ohm tap, and <0.05 ohms from the 2 ohm tap. All of those numbers will result in the amp essentially behaving as a voltage source with this and most other speakers.
So I would expect in this case that interactions between amplifier output impedance and variations of speaker impedance over the frequency range are very minimal contributors to the sonic differences the OP hears in going from one tap to another. The major contributors would seem likely to involve how the sonics of the amp itself (and perhaps also the amp’s maximum power capability) are affected by differences in loading of its output stage that depend on which tap is selected.
Regards,
-- Al
P.S: Bruce (Bifwynne), great to see you posting again!