The scientific method requires a theory followed by testing, with reliable results to prove the theory. Without testing, measurements are not necessarily significant. You need a real life A/B comparison.
Tubes, tone, and old age.
FWIW, just a short story about how things can change. 30 years ago I had two high power tube amps with SED KT88's. Bought a backup set (or two). Sold amp. What to do with tubes. Nothing, I just put them in a big drawer. Pulled them out every few years and always found them powerful but a tad bright. Never got an amp which served them well and I kinda forgot about them.
I got old. Needed new KT88's. What the hell, I pulled out the old SED's. Interestingly (to me at least) the upper frequency retained its clarity but what was bright in yesteryear, now matched the natural loss of some high frequencies in my hearing due to age. Hog heaven!
Point, if any, to this story is when you go out to buy tubes don't forget that the sound of tubes can and will vary because of the hearing of the seller as well as his knowledge of how they will sound in your system.