I've owned both MAG600 (narrow version) and ABM600 but not at the same time. I much preferred the MAG over earlier ABM heads; it sounded really punchy with lots of clarity and low end perhaps due to the less complicated signal path. The ABM600 will give you extra features but if you don't need or end up using them it might not be an upgrade at all. I've no experience with RM series.
PS, I find 340Hz to be the area that I cut to remove boxiness.