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

I am sorry, but neither Pass nor Curl can compete at the top level. Perhaps they are doing something wrong.

:)  Nelson's SIT amplifiers are some of the best sounding out there. But they are only available in kit form IIRC. SITs went out of production a long time ago.

Regarding John Curl, isn't he the one behind the Blowtorch preamp? That is one of the very few solid state preamps that has developed a legendary status.

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...?

Even though they are beneficial in many ways they are still distortions. This is why there is that term 'euphonic'. I've often felt though that distortion harmonics should be weighted according to how important and audible they are to the ear.

It was a sign of the times that Curl et all went to Parasound about 20 years ago and abondonded Blowtorch preamp and the Bar B Q amp which were very high end components. I participated with Curl and Crump (TG Audio) the last year they exhibited the BAR B Q with the Blowtorch. The following year 2001 the JC-1 (scaled down Bar B Q) debuted with Blowtorch and Entec Number Cruncher DAC. I isolated the Blowtorch, Entec and four (count em!) JC-1s that year with a one of a kind double decker Nimbus and four Promethean isolation bases.

I suspect that just like loudspeakers, when the amplifier designer engineers an amplifier to fit to a certain price point, he is also targeting an amplifier sound that he thinks best presents music as realistically as possible.  In other words, its not all about perfect square waves, SNR and THD.  Therefore, its reasonable to expect that different designers have different ideas of what sounds right...hence amplifier will sound different.

As Blindjim said....just try a few amps and it doesn't take long to see that either the amps themselves sound different....or the way they interact with the rest of the equipment results in a different sound.

Right now, I'm "trying out" a new amplifier called the 2Cherry from Digital Amplifier Company. Its class d, no modules, all in house designed circuitry using discrete components...and boy does it sound good...open, three dimensional, detailed, liquid...I could go on....for less than $3k....so I think there may well be room for continued amplifier development.

And in a way, I agree with Atmasphere...most of us say that we want as little distortion as possible...but I think we really want is what sounds the most like trumpets, pianos, guitars and voices that we have heard live...and maybe sometimes, to some listeners, a euphonic presentation is more pleasing.

Which component is further along? Speakers or Amps? Would the best speaker, cost no object, in the world show any shortcomings of the best, cost no object, amplifier in the world? Or vise versa? 

Alan Shaw would say there is very little new in amplifiers. Indeed, amplification technology is widely used and optimized for many purposes, not just audio, and is more or less a "solved" problem. For those of you who haven’t had the pleasure, this thread will show you some of his thinking. https://www.harbeth.co.uk/usergroup/forum/subjective-soundings-your-views-on-audio/electronics-sources-stands-cables-accessories/amplifier-discussion/2390-the-last-words-on-audio-amplifiers-jan-2015

And if you disagree, and can prove an audible difference under his conditions (gain matching), he's offering a pair of his top-of-the-line speakers (see the first few posts).