I've never been fully happy with dynamat facing a driver, and if excess HF is an issue behind a driver, I just use fiberfill.
Dynamat helps kill the slight vibrations that contribute to treble peaks and (mildly) poorer sound quality in most headphones, so after adding the stuff, the sound usually becomes tighter, clearer, and even more detailed to some degree, depending on the headphone. Bass extension may improve, soundstage imaging may also improve. It didn't have any effect on my Momentum on-ear or K553, example, but K553 did need fiberfill treatment instead to reduce cup reverb.
Anyway, I never tried removing the "spider" and its foam, I might try it after reacquiring HD650. I don't like the idea of brightening HD650 at all, however. I used as much Dynamat as UntilThen, and loved the improvement a lot. It tightened up the sound, widened the soundstage, and removed the bit of harshness in the lower treble. Replacing the stock 1/4" plug on the original cable with an Amphenol 1/4" improves detail, control, and clears up the bass and treble. Subbass starts coming through so tight and clear, then the Dynamat improved things more. Alternatively a complete recable, but I wasn't in a mood to build a whole cable at the time.
I wish I could get rid of the reverb that HD650's housing adds to music. My friend heard HD600, said the reverb sounds like a "train station" to him. Nicely put. Closed-back headphones often suffer from some kind of reverb that sometimes can be described as a "concert hall" effect, to my ears. Dynamat on the back of the driver only partly alleviates the issue for HD650. I'm thinking that the remaining issue is the chamber housing the magnet and diaphragm themselves, it might be what needs a little fiberfill, but getting in there safely without hurting the diaphragm is probably not feasilbe.
