The JAR660S is a great sounding headphone...I have not been listening to it recently since it is making the rounds with members and is going on tour, but below are my quick impressions and a place for more user experiences. I am a fan of the stock HD660S and think that it was a nice change from the HD650 and a bit of a lovechild of the HD580 and 650, not outright better than either, but a nice combination of tonality, speed, control, etc. The fact that it was easier to drive was a nice bonus, but still its not so easy that I feel a phone does any justice to the sound and that an amp is still needed. With this in mind, I figured the JAR mod couldn’t make it worse, and I am glad to say It is better. The JAR660S should still be considered a love child of good traits of the 650 and 580. The tonal picture is fast and accurate with low end control and presence. The upper mids/lower treble are forward leaning which keeps the engagement factor high on these. I get a wider sense of soundstage than either of the other two and detail retrieval is high up on the list of noticeable changes. Bass presentation is better defined and cleaner than the HD650, but more present than the HD580.