We were comparing phones from behind the same uDac-2 behind the same external sound card. If I remember correctly the Denon D2000, D5000, Senn HD5x5, HD600, and a pair of Grados were involved. Best Monday ever. Anyways for me it was Denon > Grados > Sennheisers. However, I wouldn't mind having a pair of HD600 as I still enjoyed them. However, I would consider 100$ on a pair of HD555 a waste.
The presentation of the music effects how enjoyable it is. After hearing a song (especially rap, hip hop, or any bass heavy rock) on the Denon 5000s the song was no longer as enjoyable on the Sennheisers.