Here are the mods that I've done, in addition to the coin mod and rear felt removal, based on @stratocaster mods from Changster. Felt ring (no change in frequency response but helped with distortion): White bloomy thing is the cotton nail polish removal pad and then there is some protective foam I pulled from the top of a mic box (different foam did make a noticeable difference in frequency reponse so YMMV): Stock vs modded frequency response: Stock Waterfall: Modded Waterfall: Stock CSD: Modded CSD: Subjectively they come across as more mid forward due to the reduction in the mid bass hump and the flattening of the 2kHz dip. However, they also sound faster because there isn't this bloat for lack of a better word presenting itself in every song. Another benefit of reducing the hump is that sub bass becomes more present as it's no longer tapering off at the same degree as the stock HD650. These still aren't perfect as I feel they are missing some sub bass and roll of the high end a little too much especially after 10kHz. They still remain the most speaker like headphone that I have heard and are only bested in frequency response by my modded HE-6. Imaging and depth remain largely the same however some things like vocals are pushed slightly closer due to the 2kHz flattening. I personally like this as I use these as my alternate reference headphones and want a totally flat sound for mixing.