If you plan on upgrading the tubes for the Tuba, I can highly recommend the NOS RCA 7189. Also, a more affordable option are the NOS Soviet Reflektor 6P14P-EV. Both are better than the stock Reissue Mullard EL84.
The RCA 7189 in particular sounds amazing on the Tuba. The sound is more 3D, better bass response and treble extension…just a much more linear sound.