How good is the McIntosh MC-2300 vs modern SS amplifiers?

They were probably good in their day, Roger, but so were many others. But you would have them today as your main amp I think not.
A solid state amp that has trouble driving a certain speaker load, can be helped with the use of an autoformer, I say save your money and get the right amp instead.

All you have to do to prove it to yourself, is put a pair of Autoformers on ANY decent solid state amp that can drive hard loads, and hear what it does to the sound compared to when used without them, even on easy loads.

All this fishing Roger, sounds to me like you may be ready to venture into solid state using output transformers?
https://d2gg9evh47fn9z.cloudfront.net/800px_COLOURBOX29710931.jpg

Cheers George

@georgehifi - sorry to say Roger won't be going down that path. He's just a curious guy that does a lot of research. When he heard Curl talk about the 2300's it got the better of his curiosity, especially given their use by the Grateful Dead with the Wall of Sound set up. So he looked into it and thought he would write about some of his thoughts and findings. If Roger was going down the path of building solid state amps, and he has built at least one I know of in the past, I'd be one of the first to hear about it.

I built many solid state amps in my youth, not knowing of the AutoTransformer Macs. But then I was just a child. I did however get a firm understanding of how transistors fail and the circuits necessary for them to be protected against shorts and reactive loads. I learned about safe operating area (SOA). I saw that at the voltages required just to make a 100 watt amplifier would intrude into the SOA of all power transisitors and still does. So the designer has to protect the transistors. I found most of the protection schemes would cause premature clipping into reactive and low impedance loads. 

When an amplifier sounds very bad on a particular speaker and good on others I contend it is misbehaving into that load. Two common examples are current limiting (load line limiting) and outright instabilitiy usually in the form of birdies (small HF oscillations on the wave that look like a bird on a wire). Birdies sound like the worst kind of clipping, yet the amp may be far from clipping. 

This concept of matching speaker and SS amplifier appears to be misunderstood and a bit overrated. As stated above the particular speaker may make the amplifier unhappy. If the amplifier is happy then one may consider impedance variations that are exaserbated by high output impedance. 

If I were to make SS amps I would like likely use autoformers because:
  
1. I know how to make them
2. They allow me to use the full rating of the transistors.
3. They allow me to use same sex transistors
4. They protect against DC faults
5. They present the optimum load to the amplifier via their taps

If I were to make SS amps I would like likely use autoformers because:

Judging by this statement, as I know your no fool, and that you have never A/B’d a highly rated solid state amp that’s happy driving into a speaker considered to be a hard load, then place the "said" best autoformers in to see what they do. You will be surprised at just how much the sound degrades.

Granted autoformers have their place with amps that cannot drive certain loads, but I say save the money and get the right amp instead, you’ll be far better off.

If you have an OTL amplifier you should know that the power is greatly reduced into low impedance loads. Even worse is that low impedance loads will overheat the tubes at high power levels as most of the power supply voltage is being dropped across the tube not the load. So low impedance loads are hard on the tubes and cause higher distortortion All of these ills can be solved by the use of a proper Autoformer.

OTL’s are the best sounding tube amps there are ( even maybe solid state also) if they are used withing their comfort zone driving speakers that have easy loads they like, as with the highly rated solid state amp above, that changes very much so once you introduce an autoformer, into the same system, I doubt that Ralph would use autoformers if there was no need to.

It all comes back to the right amp for the right job, if not an autoformer can be a "band-aid" or "interim" fix, but an expensive one.

Cheers George

Can we agree that good amplifiers drive a large number of speakers very well and changing to another "good" amplifier will drive the chosen speaker just as well? These amplifiers have certain common characteristics that make them sound very similar on a wide variety of speakers. 

Can we further agree that some amplifiers have (rather undesirable) measurable characteristics that will make a given speaker sound uniquely different on this amplifier. And though we might like the coloration this combintion produces, it is not what the speaker designer had in mind? 

If the right amp has to be found for just some one speaker, I would say that is a bad amplifier waiting to find a home. How is one to find this perfect match?  How many of these colored amplifiers would one have to audition? Take the current OTL amps which all have high output impedance. They can properly drive less that 10 % of the speakers out there. How about an OTL that can drive a wide variety of speakers properly?