Depends IMO.
A Black Face Bassman with matching original cab, with orginal speakers in functional condition, with all the hardware in decent enough shape would be a very fair trade.
And for the record, I gigged in a fairly loud rock/punk band with a 80 watt Sunn. No reason you couldn't gig a 50 watt Bassman with some players with some self control.
As stated before, they're pretty decent guitar amps too.
Pre CBS doesn't mean much in amp land. It's Tweed (wide and narrow), Brown, Blonde, Black or Silver Face to determine value.
If it was a narrow panel tweed Bassman, I'd trade in a heartbeat, and run to buy a lottery ticket!!