MAC Autoformers?

Now there are even CAR stereo amps that can run .5 ohms all day at negligible distortion.

The problem with not using transformers, even with solid state amps is that 'negligible distortion'.
It is the mark of a good engineer to know what is negligible and what is not.
The slight amount of distortion made by most solid state amps is not negligible. The reason is that the distortion is composed almost entirely of higher ordered harmonics, and the human ear is tuned to these harmonics in several ways, a sort of convergence.
First, there is Fletcher Munson- the loudness curve. If you take a look at it, you will see that the ear is most sensitive at birdsong frequencies- up to 7KHz. This is why alarms are higher frequency. There are a lot of instrument fundamentals that are a lot lower- in particular, instruments that are near 1KHz will have a 7th that high- but here's the tickler: the 7th is also one of the harmonics that all solid state amps have in common (which we've known since the 1930s imparts a metallic quality to the sound), and the ear is insanely sensitive to this (moreso than good quality test equipment), because it also uses that and other higher orders to sense sound pressure. This is why solid state sounds bright and harsh, its why tubes are still being made and why we argue about tubes and solid state endlessly on the internet.

So this is a problem, but actually an easy solution is to simply present a higher impedance load to the output section of the solid state amp. Right away it will make less of these higher orders and so will sound smoother and more detailed. That's what the autoformer is for!
One other point- its not to anyone's advantage to make **any** amplifier work hard! You can know right away that if you do so, it will have higher distortion. In this regard, if the most realistic audio reproduction is your goal, your amplifier investment dollar will always be better served by a speaker that is higher impedance- 8 ohms or more. There is little point to 4 ohms unless sound pressure is your goal rather than sound quality.

I'm by no means an engineer, but very much appreciate the technical descriptions of why some amplifiers are designed with autoformers.

One comment about McIntosh owning the patent.  The patent on that invention expired long ago.  I think the more viable reason more amplifier manufacturers don't use output transformers for solid state amps is the cost and how much weight it adds to the unit.

I spent about a year visiting all sorts of shops to shop for a new pair of speakers.  The pair I finally selected (Focal Sopra No2's) match up with the MC452 "quad balanced" amplifier from McIntosh.  I've heard those speakers driven by amps which were called "fast" or "more detailed" (higher damping factor?), but the Mac (to me) just sounded more "musical" than the others.  Could it be the autoformer?  Maybe.  But I wouldn't sweat it if it was not!

There is a reason why they retain value, and I really don't believe that it is the blue meters. They sound good, and from my experience, the amps with autoformers have more authority and "gravitas" I guess I'd say.

The only negative with Mac autoformers is in doubling the amplifier cost and weight.

I have been looking at integrated amps and when researching the Mac I noticed the only ones without auotformers are the lowest rated at 100w. I wonder why that would be the case are they not needed in the low watt amps? I assume they could use them just as easy in a 100w integrated as a 200w integrated. I belive they use the autoformer in all the new power amps even the lowest 150w mc152. I don't own one or know  that much about it just curious why only the low watt amps don't have them ? Could it be they are meant to be used in smaller rooms with easier to drive speakers?