Thanks G, I just found an ABM600 EVO IV that I can get for roughly the same price, should I just go for that even though it's got less wattage than the other two?While I have never played through a Rootmaster 800 EVO or an MB800, I did own an MB500 and Rootmaster 500 EVO. While the Rootmaster 220 and 420 were underpowered, the Rootmaster EVO series is not. They were redesigned with more volume output and just by using my ears I would say the RM500 EVO is probably louder than the MB500 although they are both in the stupid loud category.
The MB series has that baked in GK low end and sizzle sound while the RM has what I would call a warmer vintage tone although by just raising the high mid and treble on it can get it pretty close to GK territory. Hit the shape button and it gets you into ABM territory.
The only reason I no longer own the MB500 is because I wanted the 700rb II instead and I hated the look of the MB500, but it also has a more clean solid tone with punch compared to how growly the 700rb II can get with certain basses. But that 700rb II is definitely one loud mother. I still own the RM500 EVO and I do like the compressor on it as it thickens up the tone and has an auto gain built in so you do not loose volume. In fact it actually gets louder with the compressor.
I have no idea which one you'll like better so I'll just add that GK's sound great with pedals, but if you already like the Ashdown MAG then you'll probably like the RM as well. Good Luck.