OP,
For 6SN7's I really like the NOS RCA red base some of the best tubes IMO of this type. they also rated to last 10,000 hours. I run them in my tube amp and am quite happy with the tubes. detailed with extended bass mids and highs. smooth and warm but not over the top. just a great performer.
mkh1099,
15 days is plenty IMO. few days to burn in and the rest to evaluate seriously how much time do you need to decide if you like something or not. I don't know a dealer who'd give you their equipment for that long either. oh that 48 hours at the factory is primarily a stress test to make sure there are no short lived components in the item.
My suggestion to you is get it home hook it up and let it play all day and night for the fist couple days, you do not need to have the amp on down stream either so it can run 24 hours a day, FM tuners work great here if you have one maybe a streamer or CD on replay. 4 days is just about 100 hours you will know by then if you will like it or no,t 8 days is 200 hours by then your fully settled and you still have a week left to evaluate.
Glen calling out people on their schiit for decades, ok that was play on words no offence.