EDIT: I've found a way to use the Ultra's Tube-Pre for a pseudo tube power amp sim with my tube preamps which now sounds as good as the Torpedo's, and here are the details...
a) I set my tube preamp's tone stack as flat as possible (typically bass and mid up, treble down, but it varies). The Seymour Duncan Tone Stack Calc for Windows works well for this!
b) I then instantiate a amp block with the Tube-Pre, and then pick a tone-stack for my rather flat sounding tube pre. I set the Tube-Pre's gain to zero, Master Volume to ten.
c) I then turn on the Tube-Pre's power section by turning Sag up from zero, and while leaving the Tube-Pre's tone stack flat, adjust the various power amp parameters to what I need considering the gain structure and tone of the tube preamp, and the selected tone stack in the Tube-Pre (I'll look at the modeled amp's power amp settings for ideas and tweak from there etc).
Leave the Tube-Pre's gain at zero and make up gain elsewhere as needed (Level, FX Loop etc, you;ll need quite a bit).
I've tried it with relatively clean through edge-of-breakup Fender/Vox, to pushed mid-gain modded Marshall/Mesa type tones with good success.
I used this technique yesterday with some of my preamps (TriAxis, ENGL E580) and I'm mind blown!
This should be on the Axe FX Wiki page or in another topic with a more appropriate title so people could find it!