I can't really do that; I have a GCX but don't want the reverb in there because it's wired to my guitar's on-board effects loop and the reverb is always on in my setup. I turn the effects loop on and off right from the guitar. Plus, I'm using all the loops.
Even if I used the GCX, wouldn't that really be the same as plugging the guitar into the reverb right before the signal goes to the preamp? I thought time-based effects are better after the preamp and before the power amp in a parallel loop, which the M4 does not have.