[Mad Max]

Yeah, I tried defoaming HD600 and it became fatiguing and balance sounded rather odd.  I didn't like it at all.
 
The sound became dull after putting the foam back, wasn't sure why, but checking the headphone more thoroughly, I found the earpads hadn't snapped in completely, and so fixing that restored the sound.

 

[Stoney]

Try again without the foam, but double check you have snapped them together fully this time.  I found the difference subtle, and mostly an increase in transparency, a reduction of "soft fuzziness".  

 

[Mad Max]

I sold my HD600 months ago.  Yes, the difference was subtle, but not subtle enough for me.  My carelessness with the attachment of the earpads was only after re-foaming the headphone.
 
The foam helps tame the lower treble, and a recable even more which helps make the headphone a pinch better balanced.