I just got my nos Amperex JAN 7308, green label from Upscale Audio. Although it is "driver" grade, it sounds very musical, an almost liquid sound. Even though I have only a couple of hours on it, it seems more refined than the 6922 EH that I have done much of my listening on. How much of this is a placebo effect, I will figure out as I burn the tube in. So far I am pleased.
Gongratulations. There are definitely different tubes for different needs, but I've found the 7308 Amperex to be perfect combination of naturality and musicality (if that makes any sense). Very refined indeed and the bass feels accurate. Not the cheapest one to get spares though ;)
Second step is to choose proper audio (certain lenght clips to loop) for listening, from different genres, so first you could make selection if tube fits better for electronic, acoustic or metal/progressive music for example. After splitting tubes by music genre you can start to compare them head to head with music genre you select, again with certain lenght clips that have elements you need to compare.
+1. I think one of the most important thing is to choose proper audio. You might need different tracks to evaluate different elements such as sound stage, bass, imaging, transparency etc. Many Michael Jackson songs are perfect for critical listening imo. They are just so nicely produced and have lots of small things going on at the same time. "Don't stop 'til you get enough" FLAC, for example, is a very nice track for such evaluation. I remember checking the wideness of sound stage by shaking my hands where I heard the sound of maracas (intro). Lots of small "bells and whistles" going on too, so it's great track to evaluate transparency.
I've probably spent around €200 on tubes. Definitely more than what I paid for the T1. It's been a fun ride but ultimately I only need few of my tubes. I'm not planning on spending on tubes anymore. I also got few great deals and, if wanted, could get my money back easily.