You can use EL84, 6BQ5, SV83, or 6P15P-EB output tubes. I have stayed with 6P15P-EB tubes bought from Decware.
You can flip the bias switch with music playing to get a different sound from your input tube. Same with the impedance switch near the speaker terminals. Input regulation tube also has an impact on sound, and you have three to chose from.
I would set up amp one way, listen for a while, then change a variable to see if you like the impact.
I would also recommend reviewing the manual and the design log in the Decware forum. Lots of insight to be had. Enjoy.