Just wanted to chime in on this since I'm now guilty of the "swap and see" approach. My Jolida uses 12AX7 as preamp tubes, 12AT7 as driver tubes, and EL34 as power tubes. I swapped my Gold Lion 12AX7 for my Telefunken ECC801S that were previously in the 12AT7 spot, and popped the old E.H. 12AT7 into the 12AT7 driver spot. That drop in gain lowered the edginess it had previously and with that perceived extra headroom it gave the low end some serious cojones for bass and kick drum and more linear depth to the soundstage. While it may be circuit dependant and against general protocol to be willy nilly, I lucked out because it sounds stupid good right now. I'was shopping NOS 12AX7 options but now I'm shopping for some 5751 or 12AT7 to replace the E.H. in the circuit now. Definitely a
for my system.
Just FYI, amp was purchased used, no warranty, and has been heavily modified. If it went tits up, it'd be a bummer but no real sweat off my back.
