Honestly, I'd compare them more to the HD600 than the HD800S. The HD800S is ambitious from a soundstage perspective. The HD600 and MM-500 aren't.
Now this isn't to say the HD600 and MM-500 sound the same -- tonally/timbre are very different. The MM-500 doesn't have the sweet seductive midrange the HD600 does. But it has a more cohesiveness to its timbre (even if it can have a slight artificial tint to it, depending on the setup), the raw definition is something I really enjoy. And unlike the HD800S, the MM-500 presents that definition in a very focused, immediate way.
But really, with the level of performance we're talking about, only you can determine for yourself if what any of these offer is important to you. It's all excellent, the question is what type of excellence do you prefer.