Thanks ZBWU. Great photos. I like your modification; I'm just concerned that the bowl shaped grille part you cut out is the major structural element of the cups. Not a problem for normal home use because you still have the strength of the driver baffle, but it will definitely reduce the strength of the headphones -- something to think about if you are knocking around with them. A simpler mod might be to just remove the outer cosmetic grilles.
I would recommend a less "invasive" modification that accomplishes the same thing acoustically. Both the inner and outer grilles are invisible to bass wavelengths. However, there is tendency for the 595 (and presumably the 555) to have some upper and mid frequency reflections -- basically as the higher frequency wavelengths bounce around inside the cup off of all the edges and hard surfaces.
I took thin sheets of acoustic foam (about 1/8th inch thick) -- you could order HD580 replacement foams for this or buy Radio Shack replacement earpads and cut them up with scissors. I applied the foam to all of the interior surfaces of the 595 cups including the inner grill and the surrounding surfaces. The only places you have to avoid are the three screw-posts and the two locating pins that stick up in ZBWU's photo above. You also have to avoid the flat shelf that runs around the perimeter of the cup from about 12 o'clock to 4 o'clock in the photo above -- the driver baffle sits flush on top of this.
To attach the foam, just lightly brush the plastic surface with a little rubber cement. It's enough to tack the foam down, but easily removable after it dries.
I started with as big a piece of foam as I could covering the inner grill and then just worked around the edges, cutting little bits and pieces of foam to fit the rest of the perimeter and the little nooks and crannies. It doesn't have to be pretty -- the foam can be pushed around to fit. Either cut holes throught the foam for the little screw posts to stick through or just work around them.
The foam damps progressively more as the frequency increases, but does pretty much nothing to bass frequencies.
As a final step, I fill the remaining volume to either side of the driver with very loosely packed acoustic poly fiber fill, the stuff used in speaker enclosures (although this step may have been gilding the lilly -- lining the cups is the big step, IMO).
The headphone improve dramatically. They still have the liveliness and sparkle on the top end, but no longer have the bit of glare or harshness on bright recordings. I have found the 595 to be excellent on vocals in general, but certain female vocals tend to be just a tad harsh. This foam modication cures that entirely. The bass is astonishing -- I think because the midrange is no longer masking the midbass. The imaging and soundstage is breathtaking. I put my hand where it think certain sounds are coming from on a recording with wide stereo separation and my hand is two feet beyond my head to either side.
I haven't listened to 555s, but this modification for 595s is a dramatic improvement.
ZBWU: I think the foam would also improve your modified phones. You would just apply it to the inside of the metal grill and the remaining outer circumference of the cup.
