I got an M50x a few weeks ago and I wasn't happy with them either. I wouldn't say they sounded empty, but It seemed like they had some strange buzzing or crackling distortion with drums and low frequencies. I don't know if they were defective or if they were just accentuating/revealing some detail of my music that I didn't like. I gave them a few days to burn in but it didn't change. I sent them back and bought a Grado SR125i instead, which I am much happier with.
Does anyone else with an M50x hear this? In the new Dream Theater album (HD Tracks version), the snare drum sounded really buzzy. I knows that's part of what snare drums sound like, but it was far less pleasant than in my Grados. And in The Eleventh Hour by Fates Warning there was a lot of crackling noise in the first minute of the song with the bass notes. I can hear it faintly in my Grados, but when I heard it in the M50x I thought something was wrong with them.