Tubes most likely to improve SQ?

Tubes will respond and sound different in different circuits, so YMMV. The best 12AU7 I’ve tried in my amps are the RCA clear tops. I’ve also been really happy substituting a 12BH7 for a 12AU7... love those.

Different el34s can sound quite different too. In my amp, the Electro-harmonix EL34s have wonderful clear mids and treble. The Nesstone EL34s have tighter bass. (since I’m bi-amping I can use both where I think they sound the best)

All of the tube slots should affect the sound, and I imagine it would be mostly in a positive way for vintage tube swaps. Yes start with 12ax7 - it’s the most likely to make the most impact, and there are gobs of them out there. Then 12au7 - like knotscott I always got big gains swapping to 12bh7 away from 12au7. When it comes down to it I don’t really like the 12au7 tube type that much. HOWEVER the 12bh7 draws twice the heater current, and your amp’s filament supply needs to have enough overhead to supply this -- it’s really going to be hard to verify this on a Chinese built amp, and I definitely wouldn’t assume it’s OK. But there are other 12au7 subs you can look into that don’t suck down much more filament juice than a 12au7. I also find power tubes make a big difference too, but the vintage market on these is extremely small and expensive. Best here to stick with Russian re-issues, and try a different compatible "type" that your amp can handle (e.g. KT88 to KT90 or KT120 is a common sub).

And don’t go too crazy spending too much on vintage NOS American / Euro tubes for a Chinese tube amp! Classic "used, tests strong" examples from the big makers (RCA, Telefunken, Amperex, Mullard) should do nicely here. The "big", well-known NOS tube dealers will charge a boatload for matched pairs -- you'll end up spending a good portion of your amp's actual cost before you know it. You could get unlucky with a set of duds from the secondary dealers/markets, and still come out ahead.

Bought Hammond vintage 12AX7A's and 12AT7's from Brent Jesse for my Jolida 302a, noticeable improvement, but biggest change was Sophia Electric EL-34's, much lower noise floor, excellent bass and voluminous soundstage, well worth the price.  Two have failed since purchased 1 1/2 yrs ago, but I often run the unit 6-8 hours per day.  I will replace the small signal tubes with Sophia Electric as well in the future. Highly recommended!

I had a pair of the VAS Citation II amps a few yrs ago with mostly the same tube compliment.  Swapping out the stock tubes made a huge difference although that will depend on other bottlenecks in your system.  I thought the Psvane 6CA7 (el34) were a big step up from the stock shuguangs and I had some Electro Harmonix which were more detailed than stock but rough in comparison to the Psvanes.   They were a bit warm with a nice palpable soundstage/imaging and at least as detailed as the EH tubes.  I had GE blackplate 5751  in place of the 12ax7.  I recall they were much better than stock but how and why elude me.  As for 12au7, I like these tubes.  I actually used these in place of 12ax7's in another amp I had and they sounded much better than the well-respected 5751's I had tried originally.  I had some Ediswans and Amperex orange globe 12au7's and both were pretty good but I found 2 which killed them (in my systems).  For a more detailed and precise sound with a leaner and quicker bass response, the Brimar 13D5 is excellent.  And the black glass Tung Sol 12au7 is at the other end and just as good.  It is warm and very holographic.  It breathes like no other 12au7 I've ever heard.  Bass is much fuller than the Brimar.  So, hope that helps and good luck.  Oh, and I did hear the clear top RCA 12au7 and they were about half way between the Brimar and Tung Sol and, for my purposes, a noticable step down from either.  Of course synergy is everything

Thank you all for your recommendations.  I will try to limit my tube buying but I know it will be difficult.