It can be done, but if you want headroom and volume you will be disappointed. Also, you will need to run it into a bass cabinet because the guitar speakers will likely blow from the lower frequencies.
I ran a Peavey Butcher head into a bass cab and it was fantastic at lower volumes. After a certain point, though, it got real boomy and useless. It just wasn't able to keep up with a band volume-wise (not that you may not find a low wattage tube amp for bass that could). One of the shortcomings, I believe, was that the frequency range was not meant for bass.