Best Preamp and Amp combination, interesting finding!!!

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.  

Unless you are dealing with analog input such as a tape or vinyl the best preamp is no preamp.  Keeping the signal digital and in the digital domain into a digital power amp where it is converted to analog for your speakers will produce the best possible sound.

All of this ignores the elephant in the room where thousands or even tens of thousands of dollars are spent on equipment while ignoring the biggest factor and that is the quality of the recording.  The producer/engineer at the sound board (when recording in studio) can destroy the result by mike placement, quality of mikes, how the signals are mixed and amplitude and tone controls of each channel mixed.  Very few engineers go for the simple but acoustically more accurate binaural recording.  They can't resist panning or boosting what they like and burying what they don't like.  Add to that problem the fact that most are listening to monitors that tend to be bass heavy or light and that influences what they hear and how they adjust the recording and you get a lot of crap and then audiophiles interpret what they are hearing without knowing how it was recorded.

I was friends with the owner of a high end usia store in Miami when I lived there in the 80s and he used to also be a recording engineer.  The work he would undertake to properly record a symphony orchestra (The New. World Symphony) to recreate the environment and the sense of having the best seat in the house was astounding.  Mikes were suspended on wires just above the best seats in the place and care was taken to preserve the sense of space and location of instruments with their natural timbre intact and to capture as wide a dynamic range as possible.  The difference between those recordings and ones where mikes are placed throughout the orchestra and mixed was night an day.

You may have noticed in listening to many modern recordings, especially noticeable in recording of symphonies where you know the instruments locations, I find it very annoying that during part of the recording the violins are on the left (usual location) and other parts, especially if it is a video recording, as the camera pans and moves the location of the instruments move.  For me it destroys the illusion of capturing a live performance.  

My son was a recording artist and I have gone into studio to observe recording sessions and seen first hand how the engineer can dramatically alter the recording as they think it is their job to put their artistic signature on the recording rather than capturing the best possible rendition of what is being played. 

Just a general two cents, based upon my research of about 4 years ago. Things may have changed since then but I doubt it.  Unless you are wiling to shell out, at least, $8,000 to $10,000 on separates, you are not going to achieve an appreciable improvement in audio quality over a quality integrated amp in the $5,000 to $6,000 range.