Of course, there are those purists who say they'd rather eat their own ear wax than DI a bass! Miking a good bass amp isn't a problem providing you're working in an environment where you can play the thing loud enough, and of course you need a decent mic. Though you could use a general-purpose dynamic vocal mic to capture the bass guitar, you'll find that most of these have a deliberate low-frequency rolloff to compensate for the proximity effect when used up close. Because of this, unless these are placed right up against thespeaker grille, you could find the bottom end tends to lose power. A better bet is to use either a non-vocal dynamic mic that has a reasonably flat low-end response or to try one of the dedicated bass/kick drum mics, placing it around six to 12 inches in front of the best-sounding speaker in the cab. If you look at the frequency plot of a kick drum mic, you'll find the frequency response looks like the back leg of a donkey that's just been hit by a truck, but the results are often flattering and powerful, which is far more important than any misplaced notions about accuracy! If you're using a valve amp, you may find you don't need to add further compression, or at least not much, but if the amp is solid-state and doesn't have a compressor of its own, then it's probably a good idea to at least try adding compression to see what can be achieved. The safest approach here is to leave the compression until you mix unless the levels are so erratic that clipping might be a problem.