Since I was previously overdamping the HP-50S and wasn't quite happy with the loud treble, I tried to look for smoother alternatives.
The frequency measurements are raw to some degree and should be taken as being indicative and nothing more.
Below is the HP-50S as it was, with two nylon discs in the cup (no stock white disc, driver blu-tacked to baffle):
Below is the HP-50S as it's now, with stock foam and one nylon disc in the cup, the stock white disc in front of the driver, driver not blu-tacked onto the baffle, and an acrylic wool-knit disc on the outside of the baffle:
To avoid confusion, the wool disc is placed like this, slightly tucked under the flaky pad:
The effect of not sealing the driver to the baffle with blu-tack I already just posted about - but basically having the driver attached to the baffle in that way seems to create ringing around 5 kHz. Taking off the blu-tack smooths out the ringing.
The effect of the white stock disc I also mentioned a few posts ago - it does a pretty good job of smoothing out the bass hump without affecting the other frequencies too much (treble changes by a few dB, but nothing massive). Note that I'm only using the white paper disc, not the thinner black one that the white disc comes attached to. I've no idea how the black disc would affect the sound.
The nylon disc in the cup smooths out the bass hump (a bit less than the white disc, actually) without altering anything else very much. My HP-50S is bass-light to some degree, so this is all that's needed to kill the bass hump completely.
The wool disc (loosely knit) in front of the baffle reduces the nasty ~10 kHz peak by about 5 dB, which is exactly what I was looking for. It also cuts the rest of the treble above 5 kHz by a few dB, which isn't a bad thing. The other result from this disc is that it pushes the pads up slightly from the middle (since the disc is tucked under the pad), which seems to enlarge the soundstage just a little bit. I tried a felt disc in place of the wool one, but that did nothing but deaden the sound.
With these mods, the sound is very good. Not quite as showy and forward as before - a bit smoother and slightly more laid-back, but without loosing too much detail and without becoming dull-sounding. Quite nice.
Edited by vid - 10/24/12 at 5:52pm