Tube vendors.

i was always getting power tubes (6550c) from TubeDepot.
All sets lasted the same with no issues.

Brent Jessee is always my go-to guy.  If you can describe the type of sound you are looking for and what your budget is, he'll provide recommendations and he also provides an optional warranty and will stand behind it.

In what ways are they better than others by "quite a large margin"? 


He does more thorough testing and has better test equipment than anyone else that I know of.

Giving Viva Tubes a shot and I bought 4 of the KT170 for the Qualiton X200. 
Should be here mid week and they had the best price by a long shot.

Impressions to come.

@invalid Thanks for the clarification. So this person has better testing and equipment than Brent Jessee, Jim McShane and the others mentioned? I've never asked such questions of my vendors to even find out what they did or had -- glad to know there are serious folks making these detailed comparisons and rankings. Super helpful to know there's one vendor way beyond the others. Thank you!