MAC Autoformers?

I have a C40 preamp driving an MC150. The C40 has an onboard 20 watt monitor amplifier. For low level listening in the evening, I use the monitor amplifier; for performance listening, I use the MC150. Both of these amps are wired into an "A-B" amplifier switch box that can instantaneously switch between both amps. On occasion, I have switched between the two trying to hear any differences. They both sound identical to me. 

I also have an MC754 driving some old JBL's for the TV. One of these days, I'll have to get of my lazy caboose and rewire to see if there's any difference between it and the MC150. I suspect there won't be much difference, if any, in the sound.

I've also demoed a pair of MC601's along side a new model MC275. Again, both amps sounded outstanding, but I heard no notable difference between the two. In that case, I'd go with the MC601's because they (a) have more power, (b) don't require tube maintenance and (c) have those lovely blue meters.

I personally prefer any McIntosh amp with an autoformer, but it's based purely on aesthetics and emotions -- not on what I can hear.

I believe Ralph Karsten (Atmasphere) has weighed in on this topic many times in the past.  I recall that Ralph has suggested the use of Zeros, an outboard autoformer, in certain instances, e.g., driving extremely low impedance electrostats.  

As to the points above about tube amps and their use of output trannies, ... the answer there can depend.  For example, my amp is an ARC Ref 150 SE.  It is a tube amp that uses output trannies.  However, ARC also uses some negative feedback to lower the output impedance and extend the bandwidth.  For example, the output impedance off the 8 ohm tap is about 1 ohm or less; about half that off the 4 ohm tap.  

What this means in practical terms is that my amp can perform "somewhat" like a low impedance solid state amp when driving speakers with varying impedance and phase angle curves, …. within reason.

I only use my amp as example.  For those interested in tube amps, check Ralph's white paper that explains the Voltage and Power Paradigms.  For most tubes amps, it is not an all or nothing proposition.  It's a spectrum issue.

BIF   

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!