Personally, didn't liked reamping directly from the interface to the AX8 without a Re-Amp box, always felt thin and not a true representation of the DI signal recorded, again just IME. Having said that, also not all Re-Amp boxes are made equal, so you may take that into account...
My best suggestion, in case you're going the Re-Amp box route is to make sure you're using the flatest setting possible and check the polarity of the recorded signal. I have the Radial JCR and it has a switch for Flat/HighPass/LowPass and I have to push the Phase button in order to have the same phase as my original reference track.
Once doing that, I think it's best to just make a loop directly into your DAW just to set the levels so the level of the DI signal sent through the line out is the same as the one recorded and then you'll be ready to reamp with the AX8, just be sure to go from your Re-Amp box straight into the input 1 of the AX8.
BTW you mention you have the Axe FX II & III, maybe there's a way to use one of the outputs to feed the AX8 and sending the signal back via S/PDIF... Dunno, I'm just guessing there's a routing option where you can reamp via USB using both units.
