copied from a thread where it was mentioned how to take apart a 668b
First off, it is very easy to take apart the 668B. The logo caps on each ear piece pop off with a sharp knife blade and the remaining disassembly is simple.
Using the AKG K240/270 velour pads, I used a closed cell foam weatherstrip approx. 3/8" x 3/8" cross section (self adhesive, graphite gray color) and ran a strip around the perimeter of the cup, under the ear pad and applied to the plastic cup. This gives a little spacing so one's ear is not resting against the driver grill. I have tried increasing this distance and it only enhances the discontinuity I hear between the bass and upper ranges, even after full break in. Increasing the distance also harms the balance in the upper mids, creating too much of a "V" sound.
Too much to go into detail here, so please PM me for more info. I am sorry for not taking pics along the way!
I use good 'ole Mortite hand-applied caulk rope to damp all the "in-between" spaces around the face of the driver grills (behind the ear pads). I appreciate that Mortite is heavily mineral filled and very acoustically dead. Other rope caulk brands are too lightweight and are not as efficient (in killing resonances). Because of how I modify the 6 vent holes around the driver grill (behind the ear pad), I could not use something similar to Dynamat. To do so would make a mod irreversible.
Except for my method of rewiring the left ear cup all the operations are simple.
VERY IMPORTANT: The internal "yoke" that allows the articulated movement and holds the driver in place, as well as other areas inside the cup, need damping. Without internal damping of these items, the entire frequency range - especially the bass - will greatly suffer. This I accomplished by using a simple formula of 1 part grout (the "filler" between ceramic flooring tiles) and 3 parts liquid electrical tape (or vinyl dip used for coating tool handles to create a better grip). I applied this with a fine, stiff bristled artist brush. Several coats are required but each coat dries quickly.
I rewired with Mogami W2534 quad microphone cable and Techflex sheathing and regret using the Techflex - it is very noisy when rubbed. Also the Mogami is very heavy for such a lightweight 'phone. Though I "cheaped out" in my choice of cable because this has been an exploratory project with no proven outcome, I highly recommend one uses their favorite/best quality cable because I have discovered the 668B truly deserves it.
I especially wish to thank audiofil for his helpful direction and sharing his experiences with the Superlux HD-668B.