A higher level than Mullard in tube quality?

I am just getting started in tube rolling which I find pretty exciting. Any thoughts or comments would be appreciated.

I have been upgrading the pre-amp tubes in a DK Design Mark II intergrated amp. My speakers are modded Magnepan 3.6's. My source is a Resolution Audio Opus 21. I listen to mostly acoustic jazz and vocals.

I went from a set of Electro-harmonix 6922 (like $20 each) to used 6dj8 Amprex "Bugle Boy" ($40 each) and that was a nice improvement -- well worth the cost. I just upgraded to Mullard 6922 NOS ($90 each) and WOW! I am amazed at the improvement. I could not have imagine that I could improve a $10,000 rig so much for so little. Now I am thinking ... what's next? Can I spend $300 and get another step up?

At this point, what is my next level up? Or is it more sideways to different type of sounds?

Question: used tubes of the same vintage are significantly less than NOS. Is there any down side to buying used (but tested) tubes? I can get 2 for the price of one NOS and probably get more hours from 2 combined. Also, it seems like a good way to lower the cost of testing different types of tubes ... or ... does the aging change the sound.

Question: I have Mullard Blackburn plant made 6922/E88CC gold pins in GE label from the 1960's. Would I get a different sound if I bought different Mullards, such as from a different time or a different plant? And do I just have to take the dealers word for the details -- it all seems so vague.

Right now I have $300 burning a hole in my pocket to try the next step up. But I fear I may hit the diminishing return ...

Thanks for your comments