Best Preamp and Amp combination, interesting finding!!!

This is the kind of thing that makes the hobby so interesting and disturbing all at once

+1 jumia! From one experiment comes a blanket generalization!

The proposition in this post contains too many variables to be worth discussing.
1.  The choice of amplifiers is based solely on retail cost.
2.  It cannot be assumed that cost is the only or even a main determinant of sound quality.
3.  The conclusion allows nothing for listeners' taste in sound, although it is noted that a nearly all listeners came to the same conclusion.
4.  The fact they did suggests that the choice of amplifiers was not optimal.
5.  We are not told if the power amplifiers used have the same output rating.  If they do not, this could affect how the loudspeakers are driven.  A $1000 amp is very likely to output less power than a $3000 amp from the same manufacturer as this is the way a range of products is built up.  How sensitive were the speakers used?  If they were underdriven this will certainly alone explain the finding.

Some 30+ years ago I and a friend conducted an experiment with amplifiers using Audio Research and Krell products.  We created a number of systems where the pre-amp was solid state and the power amp valve driven and vice-versa.  We universally found we liked the rigs with valve pre-amps a lot better.  Since that time my two systems have always been run that way.

Similarly, until I heard the van den Hul 'The Grail SB' I had always preferred the sound of valve phono amps, a series of Audio Research.
Looking for a big upgrade I found The Grail a lot better than the AR Reference and the other high end pieces I tried in my system.  Go figure!

Seems to me the only easy way you could make the amp have the greater effect would be to use a passive pre with all candidates. At least that's what I would do, unless of course I already owned my unconditional, forever pre, in which case the only job would be finding the best match for this "lifetime" component when playing the sorts of music I happen to prefer. 

Maybe too simplistic, but nothing above makes me suppose otherwise.

There is a good YouTube video from PS Audio online. And he admits that he has no technical or science based explanation why pre-amps can improve the sound. Of course everything crappy made part of the chain can mess up the sound. But this is the same for every part in the chain.

But for improvements there is no other explanation than that preamps add a coloration to the sound. They add something to the original sound signal and we get tricked because we like the more colorful better than the perfect neutral signal.

This is the same what speakers do. They add a color profile to the sound. There is no single speaker out there who is reproducing the neutral signal. 

Hifi manufactures are creating a sound signature a put some flavor to it. Basically they trick us. And sometimes it’s cheating in the best possible and welcome way.