So, I tried the ART TUBE MP with HD500x and I must admit that it makes a big difference.
I just drop the TUBE MP into a pair of patches that I use daily, no special fix to the patches itself (one high gain, one clean).
The chain goes like this:
guitar > HD500x pre-amp effect (tubescreamer, noisegate) > HD500X amp with cab sim and mic > FXLOOP > TUBE MP > HD500x mixer > HD500x effects (reverb, delay...) > L2T
The sound gains fullness, and loose that "digital factor" that you surely know.
While playing, I use to switch between FXLoop on and off to hear the difference and, believe me, I can't play without it anymore!