Bass player and a sound guy here. I do not like most built in DI's on amps, (even my own), there are too many variables, mine has pre/post EQ and even a level knob. To me the whole point of the DI is an un-effected direct feed! Plus, I've never heard a "built in" DI that sounds any where near as good as the high quality boxes I own.
So I will ONLY use a DI box or a mic. When I'm running the board, I will use a mic for players like you, who strongly feel their amp is key to their sound, but I much prefer the DI box even though I know tons of you guys on here on gonna chime in about the needing the mic!!!!!!! You have to run sound alot to understand this, the bass is among the hardest sounds to get correct in a live environment, and no matter how much your amp is used for your sound, the bass HAS to be EQ'd to the room and stage mix, your sound may be killer coming out of your amp, but it can be very easily lost in the mix, especially in rooms with poor acoustics and most bars/small clubs/average music venues have bad acoustics. The DI is more reliable than a mic because the mic gets bleed and is a more difficult feed to work with. However, I don't mind using a mic to make the bass player happy. I also never-ever want two bass feeds (mic+DI) in a live setting, it's already the hardest track to get right, the last thing I want is to have to deal with two feeds.
On a lighter note, but oh so true, it's ALWAYS the Ampeg guys who are concerned about using a DI!!!!! Don't hate me, I've played nothing but Ampeg myself for seventeen years!