Eric writes: "ive got a '59 fender bassman reissue.. like it a lot.. don't love it. but i do like it a lot. recently a real good harp player told me to change the preamp tubes,, from 12ax7 12ax7 12ax7 to 12au7 12at7 12ay7" Is that from left-to-right or right-to-left as you look at the back of the amp? Thanks