I was chatting with a serious tube guy the other night and telling him about the rolling thread here... The first question he asked was WHY? Rolling tubes without the knowledge of how they operate and without doing the calculations and adjustments to the circuitry is basically pointless.
I told him we were looking for the best sounding tubes for the amp and he corrected me by saying that without doing any of the calculations and modifications we were just finding the "Least worst compatible tubes"...
Much truth to that and something to think about.
Well, one could argue that in rolling through the EF91, EF92, EF95 and 408A tubes, collectively speaking, we tried all the "compatible" tubes. And not satisfied, we decided to try everything and anything that looked like it might work, to see if we could find something better. And at this point in time, the so-called "least worst compatible" tubes I have been rolling recently are far-and-away better than the "compatible" tubes. (But of course, I should insert the standard disclaimer, "This is with my ears and gear".)
This seems to me to be the old "measurements vs ears" debate.... and I freely admit my bias.... I trust my ears, even though they are old and well-worn....
"If it measures good and sounds bad, it is bad. If it sounds good and measures bad, you've measured the wrong thing" - Daniel R. von Recklinghausen