Been doing a little research re. tube subs for my Morrows. http://www.vacuumtubes.com/12ax7.html describes the variations on the 12AX7. Looks to me as though the 6681 would be a preferred sub, followed by the 12AD7, 12DT7, 12DM7, 12DF7. I am wondering if the higher gain of the 12AX7A might create a problem in the circuit design? Your experienced thoughts, please? Thanks! & 73, doc kd4e