the modder in me is screaming that you have stumbled upon something that needs further exploration. This type of effect was not uncommon in ortho tuning and just required a new approach to both the mechanism and degree of tuning/damping/sound shaping ( call it what you like ) I often used the term "differential damping" to describe a way of applying certain tuning to one part of the driver but not the other. As an example - does a piece of this foam with a center hole cut out of it preserve the detail, perhaps the damping material merely needs to line the perimeter of the frame ..... so many options. 

I agree with Don. The mod is fun on initial listen but it constricts the soundstage and stifles the HD650's imaging. Bass is brought to the fore with added slam inside the cups. Some will enjoy this aspect but it's flabby and less refined, congesting the entire presentation and smothering detail. Back to stock for me.

Yeah, I agree this mod changes soundstage but not too much. The approach is to tighten bass, push vocals more upfront and have more (slightly) sparkle.
I would call It bass head mod    (not quantity but quality) 

But, (and the greatest but ), bassy headphones tend to colour classical orchestra instruments too much, with this mod I have more neutral Bassoon and Oboe tone, String instruments have more edge, till not ideal but very good.

I assume no headphones or modded headphones are ideal... everything has its own flaw.

Got a hold of some hd650s (Silver) and just got to try the mod. I don't like what the foam behind the drivers do. It constricts the sound. Feels a little more canned and less free sounding. The defoaming without putting the foam at the back does sound a little better than the stock cans though unlike the hd600 where I prefer everything in stock form.

OT: I also got a hold of some hd595s and I like them better overall vs the 650 even though they are a little too bright and less controlled. Acoustic guitars sound fabulous on the 595 while a little dead on the 650. I think I may just get the 598 based on the difference of the 598 and 595 graphs having less 10khz energy on the 598.

I have HD598 also, and you are right, they are more sparkly have significantly less bass (but very well defined), and are slightly grainy. (you will notice it occasionally if you compare side by side with HD650) I spent nights to compare HD598 and Sony MA900, Finally I sold MA900     

This hd650 listening has just confirmed that I like the hd600 much more. The hd650 feels too much like listening to speakers from double the distance from the speakers than they are to each other.