So, I'm looking to buy spare tubes for my Bassman Ten just so I have them. I didn't notice it when I bought it, but the person who originally owned this amp (combo unit) has two different tubes in it, they're not both the same. My model is dated 1977. Facing the tubes from the back (looking straight at the back with it in front of you), the tube in the left slot is a GT-6L6-S. The one on the right is slightly smaller, a GT-6L6-B. Is this a problem? Should I be buying both the same tube, was this done to give it a different sound (and isn't a problem), or is this just how you're supposed to buy them for the Bassman Ten?