I just scored a pair of HD600 for a great price so I brought them to the studio today to compare them to my trusty HD650. I used Leftfield's "Bad Radio" for this listening test because as well as being a great mix, it has lots of low end and bright snares, deep vocals and wide stereo backing vocals. Played from Spotify premium on high quality through Apogee mini DAC for DA and headphone amp. I'm not going to do the whole "head-fi" type review but my impressions are that the HD600 sounds faster with better reproduction of snares and hats. The graph below will show it has more 5k and more above 10k than the HD650. I have read many reviews from so called audiophile experts that they are very similar which I didn't find at all. Maybe, as a professional engineer, I am listening more to "EQ" than the average audiophile. The HD650 certainly have more low mids and a bit more low end overall but miss the high mid snap and have less content in the 3-5k area where getting the balance between present and harsh is crucial to a mix. I was planning to keep either one in the studio and bring the other home. After this test the choice is easy. HD600 stay in the studio where the high mids and flatter response are extremely important and the HD650 come home where the slightly darker and smoother sound is great for just relaxing and getting sucked into the music.