I am guessing you have an Onix sp3. I found also that the 12ax7 position has the most effect on you tonal balance and flavor of the sound. The sp 3 has enough gain to sub a 5751 in this position and some of these really sound nice. I really like the GE black plates in this tube. In the 12ax7, I like the Groove Tube 12ax7m, but these are really hard to get. These bring back some of the classic tube sound to the amp. Also, get yourself a quad of Valve Art 350B. These babies make this a different amp entirely.
