I would guess that that a Mac with autoformer will sound good => better => best depending on the speaker load. A very demanding speaker load, the autoformer will be of a benefit, therefore, in theory, better sounding. A speaker with a very benign load will sound better when used with a non-autoformer amplifier due to the simpler topology. Or, you can listen to both amplifiers and pick the one that sounds the best to your ears, but that makes for a very poor forum discussion.
MAC Autoformers?
Someone is selling a MAC MA6500 Integrated claiming its superiority over the Ma6600 due to the fact that "it does not have the degrading autoformer design found in the MA6600". That is the first time I've heard a claim that the autoformer was a hindrance to better performance; I thought quite the opposite. What do you MAC Maves think?