How do autotransformers affect sound?

Is the only reason many other [tube] amp designers don't do this because the autotramsformer is another component in the signal path? What is the trade off?

The big tradeoff is cost. And weight. Which is the same as cost. Oh and did I mention price? Sure there are technical disadvantages, but those can be fairly effectively solved by more weight, and/or cost.

The modern Mac solid-state amplifier may not be everybody's cup of tea, but they are standing on extremely solid technical ground with their use of an output autoformer. There's no reason why they should have flabby bass (i.e. they have good LF response and pretty high damping factor) . . . I just think that most SS amps have thin-sounding bass, and this what many speakers are well suited to.

The cost/benefit equation is probably different for McIntosh as well, because they manufacture the autoformers in-house. Rumor has it that the winding machines they're using today were specially built by Frank McIntosh and Gordon Gow when the company was founded . . . and they've always had autoformers in their solid-state amps. This means that all the costs required to build the tooling, and recruit, train, and manage skilled people to do the work, is already fully depreciated.

For another company that would have to invest in this or outsourse it, it's hugely, vastly cheaper to build a direct-coupled amp to the lowest expected impedance . . . and there's also sound logic in the idea that there's better places to put manufacturing resources than into an output autoformer.

Distortion. Transformers are notoriously non-linear devices. They work great in power supplies where distortion has no impact as all you need is to step up the amperage and step down the voltage. It is very very expensive to build one that is as linear as can be easily achieved with active SS circuits.

Why is this? A transformer is wire wrapped around an iron core - so you need an understaning of magnetic properties of materials to get a full understanding but hysteresis, core saturation and geometry issues all contribute to non-linearities of transformers.

Tranformers are being suplanted in power supplies too...by high speed switching transistors.

Its a fact that when you ask a transistor amp to make more current (drive a lower impedance) there is an associated degradation that has to do with a capacitance multiplication that is occurring within the output devices themselves. This causes the amplifier to take on a harsher character, something that transistors already have a reputation for.

IOW if you were to drive a 16 ohm load as opposed to a 4 ohm load with a transistor amp, all other things being equal the amp would sound better on 16 ohms.

Steve McCormick noted this recently in using a set of ZEROs (which are an outboard autoformer) to drive a 4 ohm load with his amps, even though his amps have never had any trouble driving 4 ohms directly. I interviewed 3 other manufacturers at CES about this and I found that quite independently of each other, there was a consensus on this point.

I suspect that MacIntosh sorted out this fact decades ago.

Atmasphere...Interesting. However it seems to me that this logic would not apply to a "digital" amp. Right?

I would imagine that most high quality ss amps can maintain Class A operation into higher impedance speakers longer than into lower impedance speakers. I am not convinced that higher impedance speakers including those that have are sensitive enough to deal with the loss of extra power, that better ss amps usually provide into lower impedances, ultimately sound better. In fact, in my experience the oppposite is more often true. I truly don't know if the use of autotransformers is a cure all, but, I have my (prejudiced, ignorant?)doubts.