I wouldn't have thought they were both 4ohm cabs, i'd expect 8ohm for the ABM combo and the same for the MAG combo as they can both have extra cabs connected. That said - I think the old MAG cabs had a handling of 200w, thus a 500w load between 2 cabs would be ~250w for the MAG, this means at high volumes you definitely risk blowing the MAG. I use a MAG210 with an ABM300 head which at both 8 and 4 ohms (an extra cab) shouldn't be able to damage it.