I'm a bit surprised that the fabric would make that much of a difference since with my HE400 removing the fabric didn't have any effect so I'm thinking the current fabric is less acoustically transparent than the old one since it made a difference to you.There is a thin layer of fabric (?) Glued to the inside of the grills. Pop off the grill, peel them off and reattach grills. Super easy to do and seems easy to reverse if desired.
For me, this alleviated what was initially an unbearable emphasis on the mids.
Has anyone tried without the grill or tried a more open grill with the HE4XX? Does it help the sound stage or affect other aspects of the sound? Also, are the cups made of metal/aluminum or plastic?