My main practice amp is a 1999 Vox Cambridge 15 hybrid with a single 12AX7 tube. I don't understand circuits at all, but have read that this tube does not work like a conventional pre-amp tube in this application. I don't know what that means for its tone, which I am currently happy with. But I read so much about using NOS American tubes, that I was curious if I should grab a 12AX7 variant when I see them up for sale on CL. Would it make an audible difference? If anyone else uses this amp and has a strong recommendation, please let me . I guess some direction in terms of whether to look at tubes with more gain or less would be helpful too. Thanks.