I've never owned or listened to any of those three amps - so take the following with that in mind.
I'm a firm believer that there is very little significant audible difference between most solid state power amps - assuming they are free from defects and have good basic specs (power, noise, distortion, frequency range being the more important.) On paper there's probably little difference between any of the three amps that you mention, and even though the MA-500 is "only" rated at 125 Watts, under most listening conditions I doubt that nost people (certainly not me) would notice a difference between it and the other two.
(In my main theatre I am currently running 5 different models of power amps, from three different manufacturers, 7 amps in total, and I've never noticed an audible difference when I've swapped them around. Under a scientifically-controlled set-up / test I might - but in day-to-day listening? Not.)
So on the one hand my inclination is that you are not proposing much of an upgrade - that you're pretty well-set as is.
On another hand, this is an Outlaw Forum, and I do like their products, and I do have a vested interest in seeing them do well (so that they will be around to make and sell me more products in the future) so what the heck, buy a bunch of new Model 200s (or one of their multi-channel amps). The price won't be hugely different from used MA-700s (I assume) and you'll have new gear, with that great Outlaw warranty / service if anything goes wrong.
And I'll take all of your used amps off your hands for a broken hockey stick, two rolls of tape, a 24 of Molson Canadian, and bottle of Crown Royal.
Good luck.
Jeff Mackwood