Tubes most likely to improve SQ?

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