I just got the M-50's too. I definitely agree that these cans ruthlessly expose every weakness of a bad .mp3 or recording. At first I thought that the high freq's were masked and just sounded... terrible. Then I realized it was the .mp3 I was playing and I hadn't heard the bad treble with my cheaper headphones. I am coming to understand how good headphones create an obsession for better components down the line; they expose all the weaknesses!
I'm surprised you need more isolation, I've never heard a full-size headphone with better isolation.
*edit* Although they sound excellent, they really tired my ears and make them sore. Something about the extreme bass and the fact they are so tightly sealed makes them pound my ears, even at very low volumes. Oh well.