Are future improvements in Amp/PreAmps slowing to a crawl?

See here for a nice video of a proper listening test that shows the methodology that is required:
https://vimeo.com/137001237
With some background discussion here:
http://www.harbeth.co.uk/usergroup/forum/the-science-of-audio/the-ground-rules-of-proper-audio-evaluation/2422-how-do-we-grade-audio-electronics-a-strategy

+1 This is just blatant nonsense. On the other hand, it mirrors and feeds on  the obsession of many audiophiles with exotic electronics. These days, the electronics are  boringly irrelevant once you reach a decent quality level (and that level is easily achieved and cheap).

Sure, you need to have enough power (and for top level speakers lots of power), and the frequency response has to load independent (within 0.2 dB for your speakers). Both require a beefy (and hence more expensive) power supply. Even so, this can be achieved at relatively modest cost if you avoid boutique brands. Look here for an example: http://www.homecinema-fr.com/forum/amplificateurs-de-puissance-haute-fidelite/mesures-ampli-yamaha-p3500s-mise-a-jour-t30056383.html

Was the ouput level matched to within 0.2 dB?

I think the engineering science to design and produce a straight wire with gain reached sonic perfection in the 1980's. The next advance was in improved manufacturing technology that made such amplifiers considerably cheaper (unless you wanted the mystique of audiophile stuff that ironically was often far worse). Right now, and I suspect even more so in the future, the technological drive will come from the need to be more energy efficient. In Europe, mandatory standards of energy efficiency are tightened for more and more products. For example, our recent vacuum cleaner that comforms to new EU standards uses only half of what the previous premium model did, and is far quieter and lighter to carry (and it sucks dirt rather better). The same is happening everywhere, and the only reason that e.g. class A amplifiers have not yet been outlawed is that there are probably too few of them around. But new TVs have to meet pretty stringent legislation. I suspect this is a large part of what is driving class D developement.