You play a 5 string right? I know with my Ibanez SR480, the Rumble 200 and Rumble 500 sound exactly the same volume differences aside. The 500 has more power and more speaker area. (I am guessing at the more speaker area, I don't want to do the math. I quit doing problems like that once I got my degree...).
I sure wish you could get your hands on a different 500 combo to see if it was something design wise or your specific unit that had some kind of gremlin...
I am thinking this was the case. The 200 should be fine though...in many of the venues I play, the FOH system really carries the tone and if I am too loud it messes up the FOH mix.