Arthur, just a couple of minor clarifications . . . bifilar winding refers to the process of placing multiple transformer windings side-by-side as they are wound around the bobbin. Since an autoformer has only one winding, it technically can't be "bifilar wound".
You are correct in the point that McIntosh has increased the bandwidth of the autoformers to the point where HF audio response simply isn't an issue, but there's actually a nice side effect in the fact that the autoformer tends to roll off before the frequencies where solid-state amplifiers usually have parasitic oscillations and stability problems. If these factors are balanced correctly, then the autoformer can actually improve the stability of the amp, without the need for a silly Zobel network or the like.
Another nice benefit is the fact that the autoformer (when combined with proper current limiting) gives excellent DC-offset protection for the speaker, without needing a relay . . . even though there are indeed a few Mac autoformer-based amps with speaker relays for some reason.
You are correct in the point that McIntosh has increased the bandwidth of the autoformers to the point where HF audio response simply isn't an issue, but there's actually a nice side effect in the fact that the autoformer tends to roll off before the frequencies where solid-state amplifiers usually have parasitic oscillations and stability problems. If these factors are balanced correctly, then the autoformer can actually improve the stability of the amp, without the need for a silly Zobel network or the like.
Another nice benefit is the fact that the autoformer (when combined with proper current limiting) gives excellent DC-offset protection for the speaker, without needing a relay . . . even though there are indeed a few Mac autoformer-based amps with speaker relays for some reason.