Front damping--maybe the damping reflects the sound back into the driver causing some driver break up or perhaps the sound getting distorted when it bounces of the already moving driver.
As for being an HD600 I can definitely believe it. Just like the T50RP mods. A partly finished headphone (or in this case, not manufactured to full potential).
Ahhh. . . so love smooth, flat, low, low in distortion response FR graphs. And the CSD is perhaps the best I've seen on Changstar. I can imagine the
attack, sustain, decay of simultaneous instruments so. . .
Right now I'm making due on modding my Slants. (Kinda embarrassing, but also a similar case to the HD600. Similar in that both are good, but not technical enough to be end end game. For the Slants its reducing bass distortion and filling and smoothing out the treble, especially the 10k slight peak and last octave drop.)
I have alpha pads with the cloth removed and pad glue on. The angle is with the stitching forward about 20 degrees, or whatever spreads the pad pressure most equally across the side of the head and requires the smallest setting. This is for as equivalent a seal on the contacting pad area as possible.
The pads inside with a stiff-ish black foam (a lot like the one that comes w/the HD600 box) about an 3/4 inch tall and long enough for the pad. Then I put the foam from dt770 pads (sacrificed em'). Then stuffed, densely, piece by piece, halves of 2 ply toilet paper. About 50 or so sheets per side. I don't really know. Just a whole damn lot. Testing sine sweep and tracks as I went. The end product was a pad depth increase of about an inch, though the pad's foam itself did compress some. The bare driver felt was exposed about an inch square.
My fingers really pressed hard to compact the toilet paper damping. Otherwise, the sound muffles the treble. Subjectively, it smoothed out the 10k-20k area, but the paper tends to decompress and the treble gets reduced too much. And I say damping, because it is damping the reflections from my head, the air pressure inside the cavity, and it absorbs stray frequencies, provides a stiff-ish resistant surface for bass to propagate. Or so I hypothesize.
My measure of success is both hearing enjoyment and lack of pain afterwards. If I have pain (and temporary hearing loss at associated headphone spikes/plateaus), I feel uneasy. So I really like clean responses.
HD600 is my next project. Thanks, Stratocaster.