That's the way you do it (play your guitar on the MTV)
AX8 works very well for reamping. I just now tracked a DI of the guitar, and the AX8 via SPDIF - the Vibrolux, a cabinet and rotary. Reamping the signal of the original guitar DI through the same preset sounds virtually identical to the fully processed output recorded on the same pass through SPDIF. No discernible increase in noise.
After recording, there was hardly anything to do for reamping beyond just copying the preset and replacing the input with the FXLoop. I routed the recorded DI track (Logic Pro X) to an Aux which was routed to Line 3 output on the Apollo Twin - connected with a TRS cable to the AX8 FX RTN L (+4db). In the AX8 menu: I/O / Audio - In 2 (FX RTN) = Left only. I adjusted the send from the DAW until it sounded the same. Didn't make any adjustment to the amp settings. Easier to match than I had anticipated. The audio quality is excellent.
Sure is fun to remix this way. I've been goofing here for a while - as the tracks loop: toggling AX8's effects on and off and messing with the delay timing for funky syncopations with the rhythm tracks. I'm in danger of getting carried away and layering these options until it's a complete mess. Note to self : Serve the song!