Trying to avoid the modeling of the m3.i like the sound of the JTM-45 in my G5n. And I'm trying to copy some of the presets of the mustang into my G5n also to play through my monoprice 15w and I don't want to run it through 2 amp Models.Yes you can do that.
You can also just run it into the normal input. Just set your tone controls to the midpoint, and keep the gain down and the master up, which gives you a clean signal.
Most guitar amps don't color the sound that much in the preamp, unless you are running it into the gain input.