If I understand correctly, the original Fender 4x10 Bassman, made a great guitar amp. Then Yamaha made their own version, the G50-410. Does that mean my G50-410 might make a good bass amp? (Low to medium volume only.)
I haven't tried those era amps but I've never played a Yamaha anything that sounded bad...so should be at the least a decent amp if that era was remotely close to the current offerings.
That depends on what you want from it. The idea of what makes a good bass amp evolved a lot between the 50s and late 60s, and it hasn't stopped since then. For example longcat and I have an Ampeg B-50R from the early 2000s; 50w solid state through a single 12" speaker. It does a decent impersonation of a B-15 Flip-Top, but tends to fall flat for more contemporary tones.