Markl,
I know you have probably checked out many woods to use in your cups, but I would suggest checking out Pernambuco (pic in my avatar). It is a high grade of Brazilwood from the rain forests in South America. It has very tight dense grain (moreso than I see in any of your pictures) and is the wood of choice for bows for violins, violas, cellos, and basses. We use it because the grain is straight and resists warping which could prove useful for resisting vibration or flex in a headphone situation. The only problem I see you might have is that the wood is protected and you most likely won't be able to get raw wood out of the country (Brasil) (there are laws against it), but it might be worth your while to at least check out the properties of the wood.
On that thought it might be interesting to look at how woods used in instrument making would perform for headphoning, such as spruce (top/soundpost of violin family), maple (all other 'white' parts of a violin), and willow (used in some new basses we have now). These woods obviously are much softer, but not in a fragile way. Stradivarious instruments use these materials and are 3 centuries old.