With a valve amp, you get the same power output regardless of the output impedance. The thing that changes is your risk of blowing the thing up. If you play the amp into an impedance that the output is not matched to, you may well let the smoke out. If it hard wired to 8 ohms, then you are only able to use an 8 ohm load. If there is a switch, then you can use other cabs and match it to suit.
Basses: Wasburn Status 4, Fretless 4 made from parts, Fretted through neck 5 made by me
Amp: Eminar bass 400 into home made cab with JBL 15"