Actually I never found the HD 800's bass too weak, it rather had too much treble to my ears, particularly out of the box. Break-in helped a lot, and finally I could enjoy it, and it was the best (most honest/accurate/natural sounding) headphone I've heard.
One day I was philosophizing about the real «openness» of its design (e.g. compared to a K 1000) and the acoustic function of the conic steel mesh. Covering the downside of the mesh with thumb and index resulted in a slight, but noticeable bass increase. It was a quite positive effect, and it was repeatable. So I supposed that one function of the mesh may be the partial suppression of phase cancellation. I also remember a statement from Sennheiser saying that they could have made the bass stronger without any problem, but decided to make it as it is now. However... possibly (inconsciously) inspired by a mod proposed by IPodPJ addressing the soundstage (which I was permanently too lazy for, since I had no issue with the soundstage)...
...I attached a small (~4 cm²) piece of self-adhesive velours to the inside of the mesh, right at the bottom «corner»:
Soon later I added a layer of black velvet (~8 cm²), meant for reducing reflections on the velours pad:
The sonic result is stunning: Fuller, more relaxed sound, and the treble emphasis now is virtually a non-issue. The bass may be increased by not more than 1 dB or the like – it's just right. I notice no sonically adverse effects.
I realize that what I hear may in fact be the very same effect as the one experienced by IPodPJ. But I think my new interpretation justifies the launch of a new thread (instead of reviving the old one), also because of the different approaches/goals.
So happy modding for all of you who think the HD 800 has even greater potential than in its original state and don't want to spend the time and the money for the search for the «ideal» amp (and source) for it! And happy new year!
(On a side note: shortening the cable makes the sound smoother and more relaxed as well; but I'm not going to repeat this recommendation, as it has already caused bad blood in the above-mentioned thread.)
Edited by JaZZ - 5/15/10 at 5:24pm