I have a '65 Bandmaster that I've had since it was new that I use for guitar. It has a pair of E-V EVM12L's which are tough enough for bass use. However, this amp just doesn't cut it as an effective bass amp, except as a practice rig.
It's only 40 watts. The similar vintage Bassman amps were just 50 watts with a larger transformer and they are marginal these days except when used with a quiet band in a smallish room.
I also have an early Dual Showman equipped with a pair of E-V 15" bass speakers. Now, that one works pretty good except in bigger venues without FOH support. But my little Ampeg PF-350 through a 1x15" cabinet is just as good, maybe a bit louder.
The Bandmaster would work OK as a practice amp for bass (with appropriate speakers) but IMO it is definitely not gig-worthy. However, it is very good for what it was designed for...Guitar.
Woody J (Jack)