This I believe counts as a test to see if people can tell the difference when vacuum tubes are inserted in an audio chain. This I did in 1990 while working at a radio station in Baltimore, Maryland.
WWMX-FM Vacuum Tube Experiment
(from the page linked above)
THE PROJECT:
The following is a summary of a project conducted in 1990 by this author while Chief Engineer at a radio station in Baltimore, Maryland. Although the radio station audio was in stereo and had a clean sound it lacked realism and depth, something that this author remembered from mono Hi-Fi systems of the 1950's. The studios and audio chain were all analog from music source to transmitter and after considering any differences in equipment configuration it was decided that the primary difference was the use of vacuum tubes back then.
Utilizing a few decades of vacuum tube experience, a project was started at home to build a vacuum tube gain controlled amplifier or more commonly called a compressor. The design was all 12AX7 triodes including a gain control stage; triodes were selected because of their second and third harmonic characteristics.
The reason for building a gain controlled amplifier rather than just a simple buffer amplifier was to gain loudness. One of the pitfalls of radio broadcasting is the fact that every station Manager and Program Director want to be the loudest station on the dial. This usually results in a lot of clipping and processing of the audio with the resulting harsh high end. Using a triode as a controlled stage requires controlling the grid bias and varying the stage gain. Using grid bias to control gain has about a 30db useful range sufficient to maintain an average level. As the triode goes farther into biased gain reduction it produces increased second and third harmonics, the second harmonic adds warmth to the audio while the third adds loudness.
The plan was to create the vacuum tube gain controlling amplifier, place it in front of the existing audio processing then reduce the processing and clipping in the existing processor. The existing processor would be limiting the audio signal only enough to prevent over-modulation without adding a harsh edge to the sound, the vacuum tube processor would then make up for the loudness.
THE RESULTS:
In order to get honest listening results to see if anyone would notice any difference, the new gain controlled amplifier had to be installed without anyone's knowledge. One night after midnight the all tube gain controlled amplifier was secretly installed in front of the existing Optimod 8100A processor. The Optimod was set so its internal input broadband compressor did very little processing instead letting the vacuum tube gain control do all the broadband processing. The high frequency clipping was also reduced.
The next day listening at home, the sound of realism that was missing could now be heard, it was a subtle difference with the addition of second harmonic content generated as the tube processor performed gain control. Listening also revealed that although the amplifier was controlling gain, louder passages still sound louder even though the actual level is being reduced, this is attributed to the extra harmonics adding loudness.
A few days went by and then compliments started coming in on how good the radio station sounded including calls from other radio station engineers. One day a music consultant once employed by the station walked in and said he was driving through town and wondered what we were doing that sounded so unique. Opening the back of the equipment rack his mouth dropped open when he saw all the glowing vacuum tubes.
Not only was this a success but a big success as the radio station ratings climbed up and beat out most other stations in listenership and higher ratings means more ad revenue for the station. There is no doubt that by just adding the use of vacuum tubes had improved the audio such that more people listened longer.