differences between power tubes will also depend on how much GNFB is in the amp. If its a low FDBK design, then you are likely to hear greater differences between tubes. With high GNFB, differences become much smaller or nearly impossible to repeatedly pick out. May also want to test a few variants of the first input tube or tubes. They can have a big effect on SQ as well.
Yes, this is why you see people saying "All tubes of the same type sound the same" and other folks praising one variant over another. My builds tend to be simple designs with small amounts of local feedback, hence I hear more difference.