MAC Autoformers?

Lots of amps use autoformers. It's just another kind of transformer. 

I completely agree with Stan. The entire point of the autoformers in a Mac amp are to allow the gain devices to behave in as linear a manner as possible by relieving them of the responsibility of actually taking control of the speakers. That's fine and dandy if you like speakers that make it a point to present the most benign load possible to the amp. But if you like speakers that actually get reactive and expect the amp to actually take control, as most speakers do, then a Mac isn't the amp for you. Because the autoformers isolate the output stage from the rough and tumble world of speakers, the amp is limited in it's ability to drive more current into an impedance dip or less into a spike, as the speaker designer expects an amp to do, and that drives the speakers into nonlinear response. 

It doesn't sound like transformers are that well understood by some. Their big failing is that they are massive inductive loads on the output devices. They don't insulate the amp from the load because things happening on the secondary definitely do show evidence on the primary. That's even more the case with autoformers because the primary and secondary are electrically connected. 
I guess they serve a purpose, but not in amps I like or build. I don't think it makes sense to put a few hundred feet of wire and a couple pounds of iron between gain devices and the load if you don't have to.