It's more of a tone thing. The 600 is technically more flat but to my ears has a lack of natural timbre that the 650 has. The 650 has a little more in the bass, which make it sound warmer. Most audiophiles are people who dispise bass and live for treble, which are the people who review the 600's as better. Nothing wrong with those people, but they are more plentiful in the audiophile world than the people who enjoy bass with their music. For enjoyment, the 650's are better to my ears.