SimAudio W10's might work well with the 20's. They work very well with the 3.6's. Remember that the Mac's don't double into 4 ohms. I understand the power stage is looking at the fixed impedance of the Autoformers, but I tried the MC252 in my system on the 3.6's and it shut down on thermal overload after a few minutes of playing at moderate levels.
I've also driven a 600w (into 4 ohms) digital amp into clipping on the 3.6's....I'd get an amp with high current capacity into lower impedances and avoid any chance of under-powering the 20's.
Rooze
I've also driven a 600w (into 4 ohms) digital amp into clipping on the 3.6's....I'd get an amp with high current capacity into lower impedances and avoid any chance of under-powering the 20's.
Rooze