My understanding is that in the F-1X, tube selection has a less dramatic effect on tone, as only half of it is used as an input buffer. The best thing to do would be buy 2 or 3 different kinds, like GT, Sovtek, Fender or Ruby Tubes, and switch them out to hear them yourself. If you find that you can hear a difference, then you might want to start delving into the expensive tubes to see if they are worth the money.