See discussion in HD650 thread starting around here. I got to check these out as a loaner from a kind member. General notes: - Mod feels robust and seems to add some heft and density to the enclosure. - I am not sure if the pads are at all modified, or if these pads are just old and squished. Can anyone comment? In case they are modified, I listened and took measurements with those pads. Pads make a noticeable difference if these could use a fresh set. (paging @Jinxy245) Sound stuff relative to a stock HD650 (with fairly new but not totally stiff pads): - Bass seems marginally tighter, faster, and better defined overall. No, it is not taken to Focal levels, for a dynamic headphone, but it's a slight and worthwhile improvement. - Bass also sounds less punchy and dynamic. Perhaps can be considered a good thing if things are better damped and controlled. However, I did somewhat prefer the slightly more lively nature of the stock pair, i.e. less risk of sounding overdamped. - Mid-bass hump is still present but sounds more subdued and yet not as well integrated with the rest of the sound, i.e. a bit more isolated of a hump. - The upper half the response sounds a bit off to me relative to stock. There's a bit more of a rise and emphasis in the upper-mids followed by more of a treble roll-off from there. Doesn't sound as tonally cohesive as a stock pair. Yes, it does sound darker overall. - The stock pair, up top, is generally sharper and more neutral sounding but has a bit more of a strain. It's a bit hard to explain, but the JAR HD650 is tonally rougher if you factor in the upper-mids/lower-treble but has a slightly more natural sounding mid-to-upper-treble timbre. I'll leave it at that for now. I will experiment with a newer set of pads and see how things compare after that. I don't want to draw conclusions until then. Nonetheless, so far my gut is telling me that plenty of Dynamat will get you about where you need to be to match this (and then just mod everything else how you want). But, comparing the JAR HD650 as it was when I received it vs a stock HD650, here are some measurements. The bass extension I will need to double check, because this honestly surprised me a bit. Then again, I did find the stock pair to sound fuller and more powerful. This could be a matter of additional damping and resonance control on the JAR HD650 affecting the perceived bass response. You can see the JAR HD650 has the slightly extra rise up till about 6KHz. After that, it's a slightly deeper drop in treble from there. While these differences are small, and likely explainable by older pads and no front damping, they do create a shift in the overall tone and timbre. As for distortion, it's hard to say. The stock pair might just be a better performer with low-end distortion (slightly). Or maybe I didn't match the volume on these exactly, with the JAR HD650 being more taxed. I will double check the stock HD650 results, but what I'm not seeing is a dramatic change in measured bass distortion characteristics on the JAR HD650. I was admittedly a bit surprised by this. CSD results suggest the already-great HD650 might improve here. I'm still wondering if the bass differences I heard would be more from enclosure resonance control rather than a change in response or lower distortion. I'm not sure. Again, it wasn't some massive difference in listening. We'll have to see how these perform with newer pads, since this could also be explained by older pads putting my ear closer to the driver. I'll try to listen and measure the JAR HD650 tomorrow with fresher pads and maybe throw some front damping in the mix too.