I've been helping Jupiter Audio Research out on the side to help them dial in their finalized mod for the HD650. I think I just packed up about 10 iterations to finally send back. It's a mountain of HD650 drivers! (And one pair of HD600 drivers, which may or may not get a mod too.) As a disclaimer, I was allowed to keep a pair of my choosing as a thank you for my testing and feedback. A loaner tour should be upcoming shortly once they get these pairs back and can make some last minute changes to send back out. I think you have to buy these outright, i.e. may not be able to send your pair in (it's a logistics and dealing-with-customers sanity issue). TBD on JAR's end. I believe it will be about $400-450 for everything, including shipping. Apparently the custom made damping parts and all the silicone don't come cheap. Final mod technical details: - Same huge damping piece on back, but filled to the brim with silicone. Tightly bolted onto the enclosure. Silicone makes damping contact around the perimeter of the enclosure and across the driver on top of the magnet. - Foam behind driver, previously in spider, has been reduced to a single, thin piece. - There is silicone filling on the top, bottom, and side portions of the front side of the enclosure, under the pads. - Front foam cutout is a little larger than seen in normal mods. Cutout is same diameter as driver in full and exposes that area as such. - Plastic protector piece in front of exposed driver, front side, removed. One caveat is that the final pair I have just lightly stuffs this piece of rear foam in the magnet opening area. What you get from JAR will actually be wedged under the big damping piece. This shouldn't change the sound in a meaningful way, so I feel comfortable talking about these as-is, as does Jupiter Audio Research. Side note, for anyone that heard this pair at RMAF, it did not have the rear and front foam tweaks as mentioned above. Sound Impressions - General frequency response is very similar to that of a newer, stock HD6XX. But that hint of strain, the mildest hint of low-treble stridency, etc. have been opened up and smoothed out. - Sounds more dynamic and controlled overall. Looseness, and that strain I mentioned, seem alleviated through said improvements in control and focus. (Sense of dynamics more closely matches that of HD650s from a few years back, since newer pairs are a little more damped and HD600-ish sounding.) - Through this refinement, details across the board, top to bottom, seem easier to pick out. - Background seems blacker, though that's a bit hard to explain and quantify on headphones vs. amps and DACs. - Bass is less bloomy. Tighter, faster, better defined overall. But note that it's still an HD650. Really, it seems to combine the pros from stock pairs and fully modded pairs, the kind without any foam behind the driver, while cancelling out most of the cons of each (within limitations of what the HD650's driver can do). It's easiest to just reiterate that these retain the balance of the stock HD650 but add a whole new layer of refinement and control and conventional mods haven't quite yet reached. Measurements NOTE: All references in measurements to "3L" or "3L Tweaked" refer to the final JAR HD650 mod. That was simply the label given on the driver to differentiate it from other iterations, and the "tweak" is what made it as the final cut. So these are all final results. I'm just too lazy to have to re-do all the images and relabel them. Frequency response results look pretty darn similar to a stock HD6XX pair. Both used the exact same pads and headband to ensure consistency. The main differences seem to be a very subtle boost below 100Hz and a very subtle softening of the treble, most notable, though still very subtle, at 5KHz. I had to double check these because of how similar these are, but, honestly, I think this is a good thing. Again, this was mostly dialed in by using just a little foam behind the driver and cutting out more of the front foam. The big damping piece does a surprising amount of work affecting how you tune the headphone. 1KHz at 90dB, on my modest rig, honestly looks pretty darn similar to that of a stock HD6XX (see attachment below for convenience). It is what it is. CSD results show some interesting changes. Now, these are just captures of one set of measurements, not averages as seen in the FR and distortion results. But I was able to consistently show improvements in decay on the final JAR HD650 mod. The HD650/6XX already does extremely well here, but the JAR HD650 goes and cleans things up a bit further. HD6XX CSDs for comparison. HD6XX distortion for comparison. Very similar. It's interesting how similar the results are between the JAR HD650 mod and stock HD6XX. CSDs are the biggest clue that we're seeing some benefits from the mods, at least on my rig. Yet, in listening, improvements are obvious, as they would be in any HD650 mod. But this one in particular probably takes HD650 damping far beyond anything I've seen with Dynamat and adhesive foam. So, it makes sense that this is probably the most controlled sounding HD650 I've heard.