foe,
I have the 518 and swapped in :; NOS Mullard 5AR4 rectifier, NOS Phillips 6L6GCs, NOS GE 12AX7s or Tungsol 5751s- they took the amp to a whole new level. I generally prefer the slightly lower gain of the 5751 to 12ax7 in this amp. I am still using the stock 845s
I have the 518 and swapped in :; NOS Mullard 5AR4 rectifier, NOS Phillips 6L6GCs, NOS GE 12AX7s or Tungsol 5751s- they took the amp to a whole new level. I generally prefer the slightly lower gain of the 5751 to 12ax7 in this amp. I am still using the stock 845s