This modding stuff is fun. Sometimes the results are a bit counter intuitive. No damping=treble and too much damping=treble. I am starting to appreciate the work that went into the 2.0 in the first place. Clearly this is a great starting point for more experimentation. Right now I am thinking of trying a shelf liner circle to replace the foam. After that maybe trying some different types of shelf liner (last time I checked there were tons).
Simpler terms:
Too much damping = reduced compliance = higher resonant frequency = treble + extreme highs rolloff. Also reduced efficiency which can also lead to bass drop.
No damping = ringing in small cavity = peaky treble.
Hard damping boundary = edge effects = time domain smearing = long decay, usually in mids/highs.
I recommend plain old cotton wool as a material. The better replacements are different kinds of wool - mostly trading availability, softness and ease of controlling density for moisture resistance and mass.
Kapok (Java cotton) is another excellent material, very water resistant with larger fibers. You should be able to get it inside some pillow as stuffing.
Fiberglass wool, rock wool and mineral wool are highly annoying to work with...
Various foams can also be used, but I find cotton to work better. Albeit it's easier to create very low density foams reliably than very low density cotton sheets.
If you want something very pressure linear and not too damping (medium density), a good quality wool rolag is probably best. Can be found in specialty shops.
Oh, and if you are really feeling cost no object, unprocessed silk is an excellent material too - silk noil. It's not actually *that* expensive either. (Mulberry/cultivated is more even than Tussah/wild.)
http://prosiding.papsi.org/index.php/APS/article/view/356 - some interesting article. So kapok is good for low midrange damping, while glass wool is better for higher frequencies...
Felt is usually strictly inferior to typical high density wool in audio applications - its only saving grace is mechanical resilience, which is of no use in an earcup.
There are many kinds of felt, so talking "felt" means nothing. Typical are synthetic polypropylene, cotton and wool; silk is rare, but might be around.
The main difference is the very short randomly distributed fibres in the felt process.
Most kinds of felt are too high density for damping applications - you want something on the order of 100 - 200 g/m^2 - and most kinds of felt are 600-800 g/m^2. (Thicker lighter is better.) Something that dense will be more akin to a wall for air pressure waves.
And if you need some specific damping, felt is not as controllable as a mix of looser fabrics.
