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

The OP and some others are missing the most important consideration of amplifier design here.  Namely, that good, well thought out design techniques have been known since the mid sixties.  That is a long time, even for a senior like me.  What has changed is the advent of more available and more affordable high quality parts to choose from.  So, it is easier ( and cheaper in inflation adjusted terms ) to make high quality amps than it used to be, but none of that matters if your intent is to "design to market"  instead of producing the best equipment you can for a given amount of dollars.  This difference is closely comparable to the difference between GM and Toyota in auto manufacturing.  One tries to make the most profit for a given class of cars, while the other tries to make the best car they can at a given price point and still make money.  The same issues are true in audio manufacturing and always will be.  The other side of this "human" issue can also, be compared to autos.  For example, those people who feel the need to spend more than others can in order to say that they have "the best" of whatever.  No one needs to spend $150K+ for the highest quality auto and no one needs to spend $150K+ for the best audio gear either --BUT SOME DO.

I think, there is no definitive answer.
On one side, the business lives from the impression that the latest design is better than the model before.
On the other side, you buy/bought brain. With some luck you own a unit done right. From design, layout or parts.
Even when you go to a more modern unit from the same designer, there is no guarantee that it will sound better. Different? Yes. But better?
After all my years with High End I think, most have done something outstanding on pure luck. They made something and it was a 10 out of 10.
The majority of Audiophiles want to invest money after some time. They want to change. And to justify the investment or to be King somewhere, they tell and think, yes, it is better than my former unit. And when it is more expensive, it has to be better. That’s the rule of the High End game.
Audiophiles, who go for a cheaper unit because it is better and talk about that, are rare.
But probably they know it better what is going on.
When i go to High End Shows the last 15 years, my impression was, 80% is for - expensive - ear cancer, 20% is good.
When everything will become better, this percentage should vary. But it doesn’t.

A couple of weeks ago, I had my Ayre amp looked at for a "noise" - "resonance" that I heard in my left speaker.  Ayre told me that the amp was performing well with no noise that they could detect.....I asked them to look further and do anything that they thought would improve the piece. I got the amp back after they replaced a number of capacitors, and other improved parts.  They also applied a tape to the heat sinks....the result was a better sounding amp...but the circuit itself wasn't changed.  (I doubt if the resonance tape on the heat sinks did THAT much). (the original "noise" prevails.)

I was wondering how long it would take for the "all cables sound the same" camp to chime in with the "all amps sound the same" nonsense.
Talk about an agenda.