I'm picking up an XS400 combo next week and I'm wondering how much of the tone will be affected by replacing the tube in the preamp. I've noticed a massive change in tone when one of the guys in the program changed the tubes in an old Fender Bassman, but it's all tube, not just a tube preamp. Anyone have an insight on this? Also, any reccomendations for replacement tubes? EDIT: The tube already in the amp is a Sovtek 12AX7A.