If you want to go the mic route, then you can't go wrong with, yes, an Audix D6. I wouldn't recommend an SM57, though-too much muddiness. Or, if you really want to shell out some bucks for ultra-high end, then try an Earthworks SR40 for no coloration and ONLY the sound of YOUR bass.
I've used the D6 on recordings, though, and it sounds pretty faithful and ballsy. I work where we sell the Earthworks but haven't recorded or gigged with them.
Remember, mic the speaker that defines your tone (if it's a multi-speaker design), like a mid-range speaker between the horn and woofer.
I think the best combo is getting the DI in there first as a foundation, and then amp it up a bit (no pun intended) with a mic. Of course, a good/fairly transparent DI is essential. The one on my GK 1001RB-II is actually pretty damned good, and soundguys like it.