I would stick to 12AX7. Sometimes the circuits are designed for them. I like Tung Sol RI in V1 positions in all my amps (guitar and bass) and JJ's everywhere else. On an amp with a slightly dark flavor like the SVT-4, you might like Electro Harmonix 12AX7's (which to my ears are a tiny bit brighter on the high end). A few people whose opinions I respect here like them in their Ampegs.
Cirrus 5 / Mesa Bass 400 / D180 / BDDI / Mesa PH Cabs