Hello Sahara! Congrats on a very nice amp
Will copy/paste my response from another thread regarding tubes for the GES;
Personally I am 100% happy with my maxed out GES with the SR-009.
However initially I was a bit underwhelmed by it's performance due to a bit overpowering and slightly flabby bass. This was quickly cured by swapping out the 12BZ7 no-name driver tubes that the GES came with and instead swapping in a matched quad of reissue "mullard" 12AX7. The bass-issue was solved with these tubes but the overall sound appeared a little brittle and thin. Better but not 100% satisfactory. Then I tried a matched quad of NOS matsu****a slanted getter aka "japanese mullards". Huge improvement over the re-issue "mullards"! Much rounder and well-defined mid-range. A smidgeon bright maybe. Next I tried a perfectly matched quad raytheon made for baldwin NOS tubes 62's long black plates.
Audio nirvana! Perfect tubes for me. My search for suitable 12AX7 tubes were over.
As an extra bonus the 12AX7 seems to run colder than the 12BZ7. The gain seems to be lower too though, the maximum comfortable listening spot with the 12BZ7 were at about 10 o'clock and with 12AX7 at around 11 o'clock with the volume knob.
For the power tubes 6S4 I first tried a quad of NOS NIB general electrics tubes instead of the stock no-name tubes. A clear improvement over stock but not as markedly as 12AX7 tube change. Pitch-black backdrop, zero noise compared to a slight hum with the stock 6S4 and slightly better soundstage and dynamics. I tried a matched quad of westinghouse NOS 6S4 tubes afterwards. I could not discern any difference between these two tubes. Both sounded very good so I settled with the westinghouse due to aesthetic reasons. Prettier glow and markings
Overall I am very happy with my set-up right now. I am loving the small size of the GES and its looks. I most likely won't change anything within the next 5 years at least. Zero listening fatigue, I can listen and enjoy music for at least 10-16 hours a day which I have also done on several days after completing my headphone rig. Everyone that has listened to my rig has been floored. Even my fiancée who thought I was crazy for spending so much on a pair of headphones thought it was all worth it after she had a listen