MAC Autoformers?

^No, usually. Typical loudspeakers vary sensitivity with impedance.

So here is the question: a speaker with huge variation in impedance curve will receive considerably more power where the impedance deeps way below its average curve

If the amp can’t supply like this it won’t sound flat from 20hz to 20khz, look at the (grey trace), this is the frequency response of the amp into a mild simulated speaker load, and it will benefit from the use of an autoformer..
https://www.stereo.net.au/forums/applications/core/interface/imageproxy/imageproxy.php?img=https://w...

An amp that can supply, looks like this (grey trace) nice and flat 20hz to 20khz and if you put a autoformer on it, the sound will be worse.
https://www.stereo.net.au/forums/applications/core/interface/imageproxy/imageproxy.php?img=https://w...

Cheers George

Shadorne, I don’t know. Careful amplifier pairing might be prudent.

I believe he meant that with an autoformer an amp can be designed with lax’ed parameters eg: that make it stable. And that an autoformer can then isolate it from the bad outside world speaker emf etc, that may make it go into oscillation or ring or whatever. This this autoformer makes this amp listenable and reliable. It's a Band-aid.

This statement is false. The reason is that in a Mac, the autoformer is included in the feedback loop (this fact is also ignored by @unsound). Further, any kind of output transformer does not provide 'isolation' as suggested above, instead it **transforms** impedance. So in this way, a variation of speaker impedance is transformed to a variation in load impedance to the output transistors (but at a different impedance as defined by the transformer or autoformer) and they respond in kind. Ringing and oscillation is a red herring - ringing is handled by the feedback, and if anything, the amp is less likely to oscillate when a transformer is employed!

For pure tube enthusiasts, the only solution is to find speakers that have flat and high'ish impedance functions (say 16 ohms) over their entire frequency ranges. I do not think there are a lot of beasts like that out there. Ralph, if you can make some suggestions, please do.  

Btw, another knotty subject that Ralph and Al have posted about some years ago is low damping factor with high output impedance tube amps. Ralph, I forgot what you posted. Care to re-educate us?

This is a bit of a digression on this thread. If others are interested in this topic, I recommend a new thread:

Actually tubes can handle speakers with variable speaker loads and do just fine- the Sound Labs ESLs are good example. Like all ESLs, their impedance varies by about 10:1 over their entire range, and tubes can usually manage them better than solid state.
With regards to output impedance, a high output impedance does not imply universal frequency response colorations. It does mean that you will have to be more careful about the match between amp and speaker, but IME this is an issue regardless of the amp and speaker anyway :) 

The elephant in the room is the fact that the ear converts distortion into tonality- and in this regard the ear has tipping points where the tonality of distortion is favored over actual frequency response. IOW, it can be more important to get rid of the colorations caused by distortion than it is to have perfectly flat frequency response; if you look at speaker response curves, the latter does not exist anyway!
The ear is far more sensitive to higher ordered harmonics than it is lower orders, by several orders of magnitude. This is why solid state amps can have the coloration of 'bright' and 'harsh' despite having very low overall THD. By not running feedback, it is possible to reduce these higher ordered harmonics if the circuit employs good design principles (in other words, has good open loop linearity). This is far easier to do with tubes than transistors! There are good solid state designs with zero feedback, but they are rare. 

An alternative to zero feedback is to use an autoformer that allows the output transistors to drive a higher impedance. In this way higher ordered harmonics are suppressed as all amplifiers make less distortion into higher impedances. The downside is that overall, the amp makes less power on account of that higher impedance. But in the world of high end, lower distortion is far more important then overall power.

Wow!  Lots of opinions here!

In my case, my McIntosh MC452 pairs really well with my Focal Sopra No2 speakers, I can listen to them for many hours without fatigue.  I enjoy this combination very much, very much the only thing I really care about.