I've been scouring the internet, and asking people for any info about biasing Fender's Bassbreaker 15 with no success. I finally broke down and called Fender customer support to ask them myself. According to Fender, the Bassbreaker 15 is indeed cathode biased, and does not require re-biasing when changing tubes. Now when someone else does an internet search they will have some info available to them. It would be nice if they mentioned tube changes in their manual, but they are apparently concerned about DIY liability, so they hold the information close and tend to point people to an authorized service center as much as possible. I must have convinced the guy I talked with that I wasn't going to electrocute myself. Anyway, for anyone who wants to know, according to Fender Tech support, it IS safe to swap tubes without re-biasing the Bassbreaker 15.