Referring to the sound level. For a long time during tube rolling, when using headphones, I had noticed that the sound level changes many characteristics of SQ. Then recently I read a note by someone, whose judgement I trust, that after adjusting for sound level (equalizing it so the SPL is the same for both tubes) the difference between tubes is very hard if not impossible to detect (my paraphrase).
Now, I don't have great ears, but I compared my best tube (TF E88CC) with my worst tubes (Shuguang 12AU7 and Mullard 6AQ6) and when comparing at the same volume level I could tell no difference. I trust that many others can hear a difference, and I thought I did in the past, but during my brief experiment I couldn't.
I will do more testing, however, when I get a chance.