You're already in it. Tube integrateds are the max value already. You are at this point down to splitting hairs. In other words nobody can say for sure as from here on in it comes down to your particular taste and requirements. Except when it comes to power. There I think a pretty good argument can be made for max power/quality coming in around 50-60 watts. KT88, 6550, maybe EL34, somewhere in there. You can go higher, sure, and lower too, but that I think is the sweet spot.
