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.
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.