The Axe-FX II is not my first modelling amp, it's my second. I found with my previous one, which, relatively speaking, is a mere toy, a Behringer Bass V-Amp Pro, that with the V-Amp it was almost impossible to get feedback like that at any volume level. The reason for this, as I interpret it, is that there is too much time delay between the input signal to the amp and the time the result comes out of the speaker. It's a barely noticeable delay so that means it's probably around 30 milliseconds.
Responsive feedback, as I interpret it, requires a very, very short stimulus-response loop time. And with the Axe-FX, it's apparently fast enough. I can get infinite feedback/sustain out of it, without even having to go to crazy volume levels to do it.
To get this happening, I suggest you start by boosting the frequencies you most want to feed back. Use a parametric EQ (pre-gain) and sweep it until it sings.