Been a long time coming on this, meaning I kept running into real-life delays (or just procrastinating), that kept me from posting my thoughts and some measurements on these. JAR = Jupiter Audio Research, this time with a modded HD600. Mod Updates, Context, and Background Please be aware, the original HD650 mod had a layer of foam sandwiched between the large damping piece in back and the rear driver/magnet, somewhat preventing the actual silicone stuffing from making full damping contact with the back of driver/magnet. He has since switched to making sure the damping piece has nothing in between it and the driver/magnet or enclosure. The new solution involves a removable, plastic piece, which you can attach to the back of the headphones (over magnet port), adhered with doubled sided 3M tape (easy to replace when it loses stickiness), with a cutout that will hold about a 2mm thick piece of circular foam. This means you can use around 1 layer of the original Sennheiser rear foam, choose your own, or go naked! The JAR HD600 in this case uses the new damping scheme. The JAR HD650 mod will now come as such too, and I think has been since he sorted this all out. I can't speak to what you might need to do to "retrofit" an older pair, nor can I say if it's worth it or not for you. JAR HD600 Impressions I was never a huge fan of the HD600. It always had too much of an upper-mid emphasis that drove me nuts. It sounded hot, hot, hot. To put the JAR HD600 in perspective, I've more or less stopped listening to any form of the HD650 since receiving it. I would say the JAR HD600 is sort of a mixture of the HD600, HD650, and Focal Clear, with its own sort of flavor and not so much of the more egregious downsides of those three individually. The low-end is similar to that of a fresh HD6XX, which I think are a little more neutral sounding than the older HD650 units. But, even then, it's dialed down just a wee bit from there. It's also just a little less thick through the midrange in general. It sounds a little tighter and cleaner than the HD650/6XX. There is still a bit of an upper-midrange bite to the JAR HD600, but it isn't hot sounding like the HD600 I had years ago. In fact, it's most akin to the Focal Clear here, having a bit of extra bite through 1-4KHz, but still not quite as aggressive as the Clear. Nonetheless, on some genres, you might hear a bit of that upper-mid sort of emphasis. I'd best describe it as a sort of high-pitched "heeeeee" sound. But it's fairly mild and not something that bothers me. A good tube amp can take the edge off, but even that is not necessary. Beyond that, the treble performance is again somewhere between the HD650 and Focal Clear. It doesn't have the bright, rough, metallic nature of the Focal Clear. It retains the nice Sennheiser timbre. But it's overall a little brighter than the HD650, especially around 10KHz. Taking this into account, it doesn't have the HD650 veil at all, but it's more agreeable to listen to than a stock HD600 or Focal Clear. Even though there's still a bit of upper-mid aggression on the JAR HD600, it's hard to go back to the HD650. The JAR HD600 is very lively, really grips the music, and has a little bit of that HD600/650 flair or coloration while remaining relatively neutral overall. I just find it to be a more engaging and tangible listening experience over the HD650. However, I do still think the HD650 reigns supreme with overall midrange and treble tone and timbre. It's more refined and balanced, but can be a bit sleepy at times. Well, I think it's more than the fatter bass/low-mids on the HD650 slow it down, which is certainly not an issue on the JAR HD600. But there are times the additional low-end coloration and more relaxed sound of the HD650 are more up my alley. Depends on my mood. I've also found that, with the new damping piece on the JAR mod, I can run denser foam on the JAR HD650 to make it a little more neutral sounding. I'll try to say more on that later. Note: For all tests, I ran the JAR HD600 with 1 piece of original Sennheiser foam on the rear side. Conclusion I couldn't deal with the hot sound of the original HD600. The HD650 can be a little too heavy handed or sleepy at times, even with mods, despite being so good in many areas. The Focal Clear improves on both of these in many ways, but its timbre isn't super great and it's a little too aggressive/bright sounding. It's also not comfortable. The JAR HD600 is very listenable, has some of the Focal Clear aggressiveness without so many nasty side effects, and has a more lively and more neutral sound than the HD650 (or HD6XX or JAR HD650). It has more or less become my main reference headphone for the last few months.