For live stuff, I like the Bass -> Amp setup. Although I have tried the Sansamp GT-2 as a preamp in front of my Peavey Tour 450, and really liked some of the tones I was getting vs straight into the amp. I also have the Digitech GNX 3000, which has some bass modeling, but didn't like it through any kind of amp for bass. However, going directly into my PA using the XLR out, it sounds great. For me, the Sanamp stuff works decently either as a preamp or DI, but I think they work better as a DI, the modeling (blanket over your speakers sound) can still be heard directly through an amp, but is more tolerable than other modeling pedals.
Overall, I really only like modelers for recording, direct to PA, low volume practice.
Overall, I really only like modelers for recording, direct to PA, low volume practice.