I very much enjoy tube rolling on my Little Dot Mk III. It seems to be a cheap way to tweak the sound to my tastes...but I strongly suspect that the differences I hear are due at least as much to expectation bias as to objective differences. It is not that measurable differences do not exist (as in cables), its just that the tube amp forums I visit always have many posts on subjective differences, and no measurements.
As a result there are strongly held beliefs on these boards that may or may not be built in fact.
These include:
1. That stock Chinese tubes invariably perform poorly compared to new or vintage Russian, European, and American tubes. This one is interesting because I have purchased highly rated vintage Soviet era tubes that appear identical to poorly rated modern Chinese tubes, my guess is the tooling was sold. Why should modern Chinese tubes be invariably worse? There seems to be almost universal belief that they are.
2. That tube brands have a house sound. Or (as mentioned earlier) are unit to unit variations greater than differences between brands. If so, then the results of tube rolling is a crap shoot.
3. In general, do vintage tubes outperform modern tubes...and if so, what vintage represents the golden age?
4. Due military grade tubes have any real advantage or disadvantage in audio applications.
5. What is the audible impact of tubes that test higher on a tube tester (as long as both are within spec).
I am sure any testing done here will not answer most or perhaps any of these questions...but if objective measurement was more widely available then a lot of myths could be dispelled.