original tubes for the Herron VTPH Phono preamp

Replacing the 12AT7 is an excellent idea. I've found that the black plate variety sounds best, I'm currently using a Brimar 12AT7. The RCA version is also good, but not as nice as the Brimar.

For the 12AX7s, I've experimented with different types in both the front and rear positions to get the most of the unit. I've found that the more linear Tesla ECC803 (gold pin) in the front is better, and am using the warmer GE 5751 (5 star triple mica) in the rear, give the best mix of warmth and detail.

Replace the 12AT7 with a Seimens ECC801S first. You will hear a lot more details after that. Then change out the first pair of 12AX7 with Mazda 12AX7 (If you can find them). They work well on both VTPH-1 and the brand new VTPH-2.
Enjoy,
Phillip