Did the mod last night. The sonic results are very good so far! These actually have a proper transition across the center of the soundstage now, before they sort of "jumped" from left to right without a clean transition during sounds that sweep evenly. Bass is definitely improved, and the lower midrange is much more articulate. The "congested" feeling has gone away entirely, and the upper mids are smoother. Female voices in particular are now much richer, I tested using Diana Krall's
Live in Paris DVD and was much happier with the "honey and bourbon" tonal quality of her voice, which didn't come through well at all prior to the mod. Once they finish breaking in, I think these (modded) cans are going to be very capable on music.
I did the mod slightly differently... in looking at the driver enclosure, I realized that the area where most people are drilling the holes sits in very close contact with the outer shell of the headphones, which may be the reason some people are reporting "chuffing" after the mod... the clearance is almost nil there. I chose to vent from the top end of the enclosure, or the "side" if you have the driver sitting face down, directly into an open void area around one of the screw standoff columns.
Pic 1, you can see where I drilled at the top end, with two 3/32" holes (2.38mm):
Pic 2, you can see the area where the holes vent, I have indicated with the red arrows:
The blue arrow points to the edge of the cosmetic shell that covers the center area of the main housing. I didn't realize at first, but the outer cosmetic shell of the headphones isn't attached by anything other than double-sided adhesive tape, which you can see in the photo is a sort of reddish-brown color.
I figured this out inadvertently when I overpacked the cotton in the middle and popped one of them free. I'm going to have to go back in and superglue it in place, it doesn't seem to want to hold well now and separates slightly at the top edge.
Pic 3, showing the cotton batting that I used to fill the void space in the headphone:
I used too much cotton between the driver housing and the shell area of the face of the main housing, as you can see in the photo... my final result has a much thinner layer of cotton in the center area compared to what you see in the photo. The most important areas for damping seem to be the void spaces around the edges, especially if you vent directly into the open space like I did. I filled in those areas pretty well with cotton, not packed tightly but enough to fill the space completely.
I am concerned slightly by the thin cosmetic shell that is "stuck" in place, it seems less than ideal construction. Once I superglue the shell in place I may use some very small/thin pieces of Dynamat to damp the housing further. An alternative to that might be to experiment with some sort of fine grating material to replace the cosmetic shell entirely, to convert these to semi-open cans? Running without the shells on temporarily to see what the result is like would answer the question without actually fabricating anything first. I'm not sure if I'll get around to that sort of experimentation or not, because these do sound significantly better already.
Anyone else experimenting?