I currently use the monitor out with the cabinet left on. It sounds great to me off or on. Off the amp sounds a little more articulate and on to me there is some perceived "beefiness" added to the tone, which I personally like.
I went for the MBritt verison and I love it. Just from the overview on the website the EM12 has round/punchy bass, and smooth mids and highs. The Red, White, and Blues has a tight bass, smooth mids, sparkling highs. I would be more inclined to think that the EM12 is a little more "flat", but I don't worry about any slight coloration in the tone. Any coloration is just warm tubey goodness. A lot of people use the Kemper because they want to play spitting image of the amp that was profiled. Im not concerned with that, I just want my tone to sound good. I think I am going to buy a EM12 and compare soon.