12AU7(17db) has lowest gain and 12AX7(100db) has highest -- so I would assume that 12AX7 is more volatile for upgrades.
12au7 or 12ax7
I recently purchased my first tube amp a Primaluna Prolouge 5, the previous owner said the tubes were at the end of their life. I replaced the factory KT-88's with Tung-Sol KT-120' with which I am very pleased. My questions is which would give me the biggest sonic benefit replacing the 12au7 or the 12ax7? Both will be replaced in time which do you think I should do first?