This is the JAR6XX. Read all about this headphone at the Jupiter Audio Research website. The idea behind the JAR6XX is similar to that of the various mass loading and filter adjustment modifications to the HD6XX/650 discussed here. Except JAR takes it to the next level. @CEE TEE sent these over to me to compare to the JAR600, which I happen to use as my daily driver (on most days where I want something neutral sounding). Here are some measurements of the JAR6XX, comparing to JAR600 JAR6XX Frequency Response CYAN = left, RED = right JAR6XX vs JAR600 Frequency Response CYAN = left, RED = right, GRY = JAR600 As we can see, @ext1 at Jupiter Audio Research tunes both headphones to a very similar target. The JAR600 is just a bit peaker at 5kH and 8kHz. However, this does not explain everything. From the frequency response, we'd expect these two headphones to sound almost exactly the same. They do not! There are people on the Internets that would tell that you frequency response tells you 90% of what something is going to sound like. This is NOT true. While frequency response is a major determinant of the sound, other factors like distortion and transient response (both successively more difficult to measure and correlate) play just as much a factor if not more! What if I told you that of these two headphones, the JAR600 was the more even and neutral sounding, and that the JAR6XX was the more energetic and V-shaped (thicker and sharper) headphone. We would almost guess the opposite from the above given the slightly higher peaks in the treble of the JAR600! FWIW, for all its imperfections, I consider the JAR600 to be the most perceptively neutral open dynamic headphone on the planet. This is why I have it around. JAR6XX vs HD650 Frequency Response CYAN = left, RED = right GRY = HD650 (stock) The stock HD650 has slightly more bass and treble emphasis emphasis with no dip at 2.5kHz.