Tube upgrade/rolling question

I just got and hooked up my first the 300B SET amp. It sounds very very good, the difference in comparison to my current SS McIntosh is not very big, but it's definitely there. I would describe it - it's in the presence of the music.
I beleave this amp can sound even better with better tubes.
Right now it has 2x made in China 300B tubes; 2x 6K7 tubes: and a 5Z3PAT. I need you input - what tubes can I use to make this amp sound even better?
All of your inputs a greatly appreciated.
:)