The effects that Nebula has on the tone is not an EQ issue you could fix afterwards, it is the content itself that is fucked and unless we find the errors that are the reasons for these alternations we won't get any closer to an optimal emulation of a cab.
I've already proven twice (I'm too lazy to find the posts where I did so right now) that a post curve EQ A/B'd to the original cab tone gives an almost 100% match across the frequency spectrum so we can safely ditch the idea that we would have to fix the fizz and out-of-balance tone with some surgical EQ in our chain. Actually maybe the term fizz is misguiding, it is the overall upper frequencies that have a very unpleasant, harsh and unnatural sound to it, very edgy and metallic sounding whereas a cab has a creamier, smoother tone there. And I am not being picky here, this is some issue that has been present throughout all the clips that have been posted so far and it is something that impulses seem to get right. That, and as mentioned before the lowend doesn't seem to be quite alright.
Basically, all Nebula needs to do is emulate given (good) impulses we have and add its dynamic features to them = pot of gold.
Now we have a more 3D and dynamic sounding result but the cab vibe itself gets lost during the process if you can catch my drift.
To me, anyone who thinks that Nebula tones come closer to the real deal than impulses...with all due respect...lacks the ability to hear the vast differences.
To repeat myself, yes Nebula programs result in a more open and dynamic tone but that's it. They don't feel like a guitar being played through a cab. I am unable to tell you if that is because people who did the programs might have done something wrong or if it is NAT/Nebula itself which needs adjusting for more accurate results.
In the end, impulses are still my go to solution for the closest emulation of cabs, even if it means I'd have to deal with more 2d and flat sounds. Despite being static, at least they get the harmonic content right.
That is just my own personal opinion. I do dig the abilities Nebula seems to offer in the long run but all I'm saying is that we're far from being at a point where we've tapped its full potential it seems. But then again, we'd need a good tone right from the start which we could use as an orientation.