I have two Axe-Fx IIIs, one is running 14.06 and the other 15.01.
I am going through the LiveGold TonePack at random picking different amp presets (about two dozen so far) and A/B'ing things to listen for differences.
Also, also am comparing to the Live Gold presets under new FM3 version 3 firmware, but that's not a totally apples-to-apples thing because the FM3 doesn't have Ultra-Res Cabs (at least, I don't think it does yet)... It's nice having a Lehle 3-way switcher on the floor to go back and forth easy. Everything is coming out the same CLRs, via the UAudio console.
Anyway, 15.01 my main conclusion so far is, to my ears, 15.01 has noticeably improved the sound of the individual string separation (such as playing chords), compared to 14.06. It sounds a little less mushed-together string-wise than 14.06 sounds (and that still sounded great). The update can makes you think the distortion or gain is slightly different between firmwares, but it's actually not to my ears thus far - it's just that you hear the individual strings in chords as you play more clearly than ever.
I'm going to keep comparing the different firmware/same presets for my own knowledge and edification, but so far (caveat: based on just 24 random presets out of 300+ sampled) I don't see that I need to make more than some touch-up preset changes under 15.01, and it sounds fantastic.
That new Jumped Bassman amp model sounds exactly like a real '59 Bassman mic'd coming out my speakers. This firmware is the most realistic one evocative of real tube amps yet -- and that is really saying something, given how incredibly good the all the Axe-Fx II firmware has been even since firmware 5.x.
Kudos the Cliff and the entire Fractal team for this milestone achievement!