Good morning Petro85,
Yes, the quest can be endless until we understand how certain combinations of tubes should be mixed to offset the weaknesses of both signal tubes or tubes further down the chain, weird thing is, I was about to recommend Brimar Black Glass CV1988/6SN7GTs ( as these are one of the most delicate, yet beautifully balanced ones I've heard to date ), but saw that you're already using them.
I've used TJ Music and Sophia 300B Mesh Plates in a fairly inexpensive Chinese Integrated I owned briefly called the Audio Renaissance LS-845 MK V, and absolutely adored them, but I'm somewhat biased towards Chinese tubes, as I merely question their long term reliability, as I've seen some last between 8 - 12 months, and thought at this rate, I'd be much better off merely paying more for tubes with better metal construction as well as glass work, because after you've taken into account if you were to replace these say 3 - 7 times in less then 6 years?, then better quality made NOS versions which would have a life spam approaching say 10.000 hours can in fact calculate into about an average of 3.5 hours use per day - everyday for nearly 10 years, then it becomes more obvious which then becomes a better investment.
And looking at the contents of your system, I'd say it deserves better made/sounding tubes built during a time where the resources were behind them, which costed millions and millions to develop - and then take into account just how many current produces of tubes, are willing to invest that much into said technology and/or machinery to match the overall quality of construction as well as sound of those designed during a time where tubes were the only game in town.
Even though some might see it differently?, even Russian designed tubes were made to last, they might not be as warm or cuddly as some might prefer!, but they are rugged, transparent and neutral as some might like, unless of course one is seeking to add sweetens to their sound to begin with?, I'm of the mindset that transparency and neutrality are were each of us should want the sonic of our systems to be!, but I'd be wrong.
Quality glass and alloy materials are in fact a thing of the past, those 50s, or in some rare cases tubes built upwards of 1963 wouldn't be duplicated sonically ever again, just food for thought.
Regards,

scar