How far have ss amps really come in the last twenty years?

@jdl57  If your Sound Labs have the original toroid backpanel, there's a tweak you can do that makes the speakers a lot easier (and better sounding) to drive! Alternatively you could get the new "Toroidal 2" backpanel that pretty much does the same thing.  If you really want to hear what ESLs do, traditionally since the Quads first appeared, ESLs and OTLs have been the way to do it. If your Sound Labs are so old that they have the original EI transformers for the high frequencies then they are already easy to drive, unless the bias controls are damaged which is very common with the older Sound Labs.

Hi,
sure ss amps have evolved mainly in efficiency, transparency, having better parts and construction (new materials). Design philosophy basically the same, music presentation different.
A 20 year old top amp then will still sound top today.

@atmasphere I just noticed your reply. I have the old style EI transformers and the back plates were sent to Sound Lab in the last few months where they were checked out by Dr. West himself. I had the panels rebuilt back in the '90s with what was the then new thinner mylar. I absolutely love the combination of the ML-2s and my Sound Lab A-2s.

@daveyf  "Nonetheless, aside from Class D ( which I also feel was, and continues to be, a step backwards ( except for the lower weight and heat issuance) ) it seems there really hasn’t been any true advances in the ss amp realm."

Comments like this are why Audiogon is slipping into irrelevance - for me at least.  Too high of a nonsense/substance ratio.

Class D is one answer to the question you posed at the beginning of this thread.  There are phenomenal implementations of this technology, and I will assert that they challenge everything that is upstream of them to be much better, from the recording process to playback.  If you have problems with good class D implementations, something else is amiss, and you are unaware of it because you have "synergistically" created a sound that YOU like.  When you replace your amp with a much more transparent class d amplifier - one with very little distortion, one which is able to easily control the impedance vicissitudes of your speakers, and one with plenty of clean power, you are hearing all of the upstream problems.  Don't blame good class D.  If you like your euphonic  concoction, rock on.  You are simply playing a balancing game of complementary distortions (I may have hit on one significant aspect of what much of audiophile activity centers around in that last sentence.) Been there, done that, am getting sick of it.

Transparent amplifiers (and there are others besides the usual class D suspects) will require a change to you audiophile aesthetic, and that is not something that, apparently many audiophiles can open up to.  But if you can get into it, you will have some interesting terrain to explore.

@daveyf "  @atmasphere  Distortion is an interesting term. Distortion compared to what? You mention that the human ear treats 2nd and 3rd harmonic distortion favorably. Therefore, one needs to question whether these distortions are distortions at all. Since we, as humans are using our ear/brain to determine what is considered as a true  and pure signal, wouldn’t it make more sense to label a signal that doesn’t include these 2nd and 3rd harmonics as the signal that is actually distorted...?"

Are you serious!??  It is not the job of an amplifier to supply anything to the signal.  The recorded signal should contain the actual harmonic structure of the timbre being captured.  The amp should do nothing more than reproduce that - NOT add something to it.  @atmasphere is just trying to tell you that one reason that amps sound different is that they distort the recorded  signal by adding their own 2nd and 3rd harmonic distortion in varying degrees.  If you are trying to behave as a chef and add your harmonic spice to suit your taste, then find an amp the has the distortion profile that you like.  If you want to hear the recording, get a transparent amp with little added harmonics and clean up the upstream equipment as well.  You may not like what you hear in a lot of recordings though.  If you have found  a way to create, by your choice of equipment, distortions that make all of your recordings sound decent, then fine.  I can understand that.

I have come to a place where accuracy is more interesting to me.  I am fascinated to hear the "flaws" in recordings and marvel when I hear record-engineering excellence.  But it does take some adjustment to appreciate the underlying performance when you have a relatively transparent system and the recordings have glaring problems.