Question for tube aficionados

I mentioned the 6SN7GTBs as between my preamp (Schiit Freya) and my amp (Dennis Had Firebottle HO) that's all there is for non power or rectifier tubes. My previous amp used 12AX7s and 12AT7s with my preference winding up as new Gold Lions for the 12AX7s and NOS Mullards for the 12AT7s.

Pretty simple, really.  Better tubes equal better sound.

For these variants, with few exceptions, you need to look toward old production.  Not necessarily NOS, though so many aspire to that.  I found you can do VERY well for yourself buying tubes others sneer at as they come up measuring as having obviously miles on their clocks.  Old tubes often play quite a long time, and pretty much sound great until they give out.

As has been mentioned the best 12AX7 tubes are 5751.  What I can add is the best 12AU7 are 7316.  Both tubes will cost you deep in the purse.  Both tubes instantly convince you that you won't go back.

If you do want to buy new production tubes, Shuguang triple mica 12AX7B aka Silver Dragon and Psvane 12AX7 and 12AU7 stack up surprisingly well

Have tried a fair sample of the triple mica 5751 black plate tubes, as well as nos mullard 12ax7's.  Have not tried the gold lion 12ax7s, but can relate that the grant fidelity psvane 12ax7 are one fine tube.

Nos vintage tubes sound significantly better than any tube made today and they last much longer. I use Mullards, telefunken and rca 12—7’s. The rca clear tops are nice and about 12$ a tube. I don’t like nor have had good luck with modern day reissues. 

You can try swapping tubes for sure but even better is to change a few of the resistors.  Carbon film, metal, shinkoh, caddock, etc.  cheaper way to go and a lot more fun!

Happy Listening.