I have a Axe III but I track bass via UAD SVT and playback with a DBX 160 and it is a killer sound. Not a big fan of UAD amp sims but the bass is killer.
I’d like to second this and elaborate a bit. I have them set up right next to each other in the studio and they are both excellent sounds. I use the AxeFX over the UAD for guitar every time. However for bass, I find myself plugging in to the UAD almost all of the time.
The sound quality is very close, but the workflow of getting a good sound with the UAD is much easier. As mentioned above, the console is zero latency so that’s not an issue. The main thing is that if you record a direct track, the UAD lets you tweak the parameters after tracking without reconfiguring and recording a reamp track. Listening solo, I like the detail in the AxeFX sound a bit better, but it takes a lot of processing to get that sound to sit in a mix well. It seems better suited to live situations. In the studio, the UAD is very easy to dial in something that sounds great in the mix.
If I want to add another 10% of polish to the track, I will record the unprocessed bass DI track plugged in the AxeFX input, with the AES out to the Apollo, and then put the UAD plug-in on that track in the DAW for the amp sound. The UAD guitar plug-ins sound noticeably better when using the AxeFX input stage as well.
The UAD guitar plug-ins still don’t compare to the AxeFx so its worth it to me to use the clunkier workflow for the better sound. For bass I find the SVT sounds on the UAD to be better for my purposes and easier to work with. I do not have the same opinion of the other UAD bass plug-ins. I have not been impressed with anything but the SVT’s.
It really depends what you are doing and what your priorities are. For quickly recording a good sound with the fewest hassles, UAD plug-ins sound good enough. For recording bass easily with a good SVT sound, I’d still lean more towards UAD. But if you want the versatility of getting better guitar sounds too, and don’t care so much about the workflow, then AxeFX is probably going to serve you better. If you ever plan to play live, AxeFX easily wins, both for sound and logistics.