I just asked pretty much the same question. I tried some loner Fatboy tubes NOS (6CA7), and I went through all the sets of EL 34 tubes I had on hand. I wound up purchasing some NOS Mullard EL 34's.
Another tube that impressed me was some Winged C EL 34's. Other than the Mullards I bought, these were the next-best sounding tubes.
Good luck,
Dan