I have an Axe-FX Ultra. You can certainly use it for bass, but the tuner and some of the effects track better/are optimized for guitar. There is one SVT model and cab sim, as well as a clean studio type preamp that work well for bass out of the gate. The other models are all guitar amps, but you can certainly tweak them to work for bass by modding both the EQ in the amp itself as well as with all of the additional EQ blocks available. I've gotten very cool results with bass - I'll make a quick video at some point. All that said, if I only played bass, I would not shell out that kind of money for the unit until they added some more bass models. Maybe I can get them to model my preamp...

Lots of folks have been asking for more bass sims on their website, and they added a bass specific section recently, so I wouldn't be suprised if Cliff is working on some. They are a great compnay with great customer support.