Not sure if anyone answered your 12AT7 vs 12AX7 but yes, the 12AT7 is drop in replacement for the 12AX7. They are both twintriodes. The 12AX7 has an amplication factor of 100, and I think the 12AT7 is like 70. Also you can use the 7025 (ala Fender) or a 12AU7 (Also used in some Fender's), but I...