so as last resort im helping out a local band and stepping in last minute to play bass for em next weekend. Im mainly a guitar player though, and i just bought a marshall stack a couple weeks ago. I know its a bad idea to plug a bass into a normal guitar amp, but what i was wondering is if i were to use my bass amp(which works as a head), and put it on my marshall cab, if that would kill my cabinet.

It's one of those things I dont wanna risk unless I know for sure either way and figure I should double check.

it is not an overly good idea, guitar speakers just aren't rated for the frequencies bass hits, so don't try it. you are much better off trying bass amplification through bass speakers(of course) or wide range speakers like PA speakers or Keyboard speakers, they are more designed for those frequencies