The best 12AX7 for that amp from my experience is the Mazda Chrome Plate made about 1963.
The EL-34 mod may be worth it...maybe not. You could do the Mazda 12AX7 and use Svetlana 6L6GC for less money
The EL-34 mod may be worth it...maybe not. You could do the Mazda 12AX7 and use Svetlana 6L6GC for less money