No offense, but you don't purchase a MarkIV to run a digital COSM modeler in front of it.. the mark has practically every tone that unit attempts to copy built right into it.
What do you mean it is not 'clear' at high volumes? As in you lose some shred/thrash to the sound? You're used to preamp distortion at low volumes and what you're probably hearing is your post amp kicking in, smoothing out and warming up the tone. You'll find that you'll want to change your knobs around for higher volumes in the EQ to keep the tone you hear at low volumes, at least in my experience.
If you want the best advice, ditch the Boss and use your tube amp like it was meant to. Even though you DONT need it, you might try a stomp box overdrive or boost to tighten up the sound but I cannot imagine a scenario where that's needed with a Mark IV. :idea: