If you are gonna mic the cab, be sure to use a mic that is good for bass. And whatever you do, don't let some fool tell you that a mic that is good for kick drum is also good for bass.
The best mic for bass (IMO of course) is the Heil PR40. It has excellent frequency response throughout the range.
What this guy said! If I see someone put a kick mic on my bass amp, I instantly ask to go DI, but I know I've got a crummy soundman so that will probably suck too.
I like the Electrovoice RE-20, and the Sennheiser MD 421 for live bass and the AKG C414 for studio.