Tube for aesthetix pandora / romulus

Hello,

I own an aesthetix pandora dac and i am thinking of upgrading its tubes:
- 2 E83CCS or 12AX7
- 2 6922 (6DJ8)

I have been advised to get mullard, siemens or telefunken nos valves

I don't know anything about tubes...

Would you have recommendation? Advises? Tips? Good tubes sellers?

Thanks and regards
G.