The M50s are stellar for the price. The Denon D2000 will bring a larger soundstage more refined treble and a deeper bass extension but the ATs hit a bit harder. The D2000 are rolling over $350 way out of budget. But wait no ones mentioned the AKG K518 LE.
The 518s make everything else pale in comparison. It's almost like listening to full range speakers + 12" balanced sub woofer. Since these were going to be for music you will appreciate the hard hitting sub bass, so rich it's gotta be fattening mids and treble extension that is so accurate you can feel the textures in the cymbal crashes and high hats amongst everything else. Electronica, rap, hip-hop, rock, almost anything modern sound so much better through these phones than the AT or Denons. ( I own both) The great thing about these are they come in under $50. They're portables that sound great directly out of iPhone. Lots of peeps complain short cord and bout the clamping force but once you listen to these you will forget the pain as the pleasure running through you ears will more than compensate.
The 518s brings a level of headphone to the competition that no one will be able to compete with at 4X's the price. No typo. The 518s are so fast, articulate, dynamic, smooth, efficient, detailed, natural sounding, and FUN! I am relistening to my entire collection as I can't wait to put these back on any moment I can! Two of my friends agree that while the ATs and Denons are great the AKGs are more musical and natural at the same time with better bass.