I haven't heard the MK2. I'd recommend the ES FC300. They're easily better than the ATH M50 in all aspects.
The WS99 is good, but it is discontinued. Not as good as the FC300 IMO.
Under $300, I have pretty standard recommendations:
Martin Logan Mikros 90: great audiophile can, more accurate than anything I've heard. Great isolation. Best closed can I've heard. Pretty neutral.
ES FC300: Great can with a U shaped signature.
KEF M500: somewhat neutral can, also a...