FraGGleR, I've been experimenting with a mod that's very similar to yours. The biggest difference is that I don't have any stuffing (no cotton, rock wool, fiberglass, etc.) in the cups and I've replaced the transpore-and-felt driver cover with a couple of layers of microfiber cloth. The bass is very good while the mids remain lush and lovely. It would be great if you could try something similar and see if you get similar results. (Getting adequate bass has been a challenge for me.)
I've tried a couple of different types of microfiber in a few different configurations, all with some degree of success. My current configuration: I used a piece of smooth lens cleaning cloth laid directly on top of the white driver paper and held in place with two-sided tape around the periphery of the driver grid. On top of this, I placed a swatch of plush dusting cloth. This is elevated a bit above the smooth lens cleaner by some thick two-sided tape (it resembles foam weather stripping with adhesive on both sides) around the periphery of the driver grid and over the lattice around the center square of the grid, such that none of the driver opening is obscured by tape. I'm not sure if this elevation is important. I did it because I had the thick tape handy.
I don't think there's anything magical about microfiber, aside from the fact that it has a dense weave that impedes airflow and reflects some treble back towards the ear. I haven't tried, but perhaps a complete layer of transpore (not a 2x2 cm^2 square) on top of felt would produce similar results?
As for why I've eliminated the cup stuffing, have a look at the first post in Bluemonkeyflyer's incremental mods and measurements thread. In particular, look at the waterfall curves and/or spectrographs and you'll notice that regardless of whether there's stuffing in the cups or not, the sound has dissipated by 200 ms. There isn't any evident ringing produced by the cup. So I've been experimenting to see if I could get away without any stuffing, since I've always assumed that the role of the stuffing was to absorb any reflected sounds.
Edited by micmacmo - 10/25/12 at 2:15pm