I just bought a '77 Marshall jmp 50 watt amp. I don't have a cab for it yet, but when it gets here, i was wondering if i could use my brothers bass cab to test it out to make sure everything works fine. Would this work and would it be bad for either? The cab is a Kustom cab 400watts @ 8 Ohms. I wouldn't be using it long, just long enough to make sure all the settings on my marshall work.
If the amp can be set to output 8ohms, then you will be totally fine.
Bass speakers aren't made for the (higher) frequencies that guitars produce, but as long as you don't crank it or anything, you should be fine.