 By the end of the decade, a JBL 15" speaker was available on special order for the Pro model.It was modified in 1957 for more volume, switching to four 5881 power tubes with the more efficient long-tailed pair phase inverter for a power increase to 80W.This "hi-powered," tweed-covered design continued into early 1960, after the other Professional Series of Fender amplifiers had made the transition to the modern brownface design.