[bpandbass]

Here is something I noticed about how earpad foam wear affects the Sennheiser HD5/6 series of headphones.
 
The HD650 earpads have prematurely worn in likely from the extra strong clamping force of the headband, causing them to flatten on the front of the pads while being thicker on the back. The foam also isn't as stiff anymore. I often have to rotate the ear cups forward so my ears do not touch the foam disks.
 
Recently I purchased the HD650's older sibling, the HD580 Precision. The earpads and headband padding were completely shot and after a failed DIY attempt to reupholster the pads with memory foam (they split at the seams), I broke down and ordered a new pair of earpads directly from Sennheiser. So now I have an HD650 with broken in earpads and a HD580 with brand new stiff earpads. This was what I noticed:
 
With the HD650, the bass became warmer and less rumbly over time as the earpads broke in, and the sound became more intimate. Whereas with the HD580, the brand new pads were much stiffer, and the treble was sibilant, the mids brittle and the bass lacking midrange punch.
 
So I swapped pads and I immediately noticed that with the broken in pads from the 650 installed, the HD580's treble immediately settled down and became more neutral, the upper mids were better behaved and warmer, and the mid bass gained quite a bit of warmth. With the brand new pads on, the HD650 seemed to gain back more of its lower mid bass rumble, but also became drier sounding. In short, the HD650 with broken in pads it became less intimate sounding, whereas the HD580 with the same pads on became more intimate sounding. 
 
I find with the HD580 and likely HD600 (though I haven't listened to one), broken in pads that seal better and have more give to them tend to make the headphones improve sonically, whereas with the HD650, this change isn't necessarily positive, as it makes the sound even more intimate, and makes the bass and midrange differences between the HD580 and HD600; and the HD650, even more apparent. Also note that the HD580 has a slightly wider headband than the 600 or 650, and as a result clamps less firmly than either. 
 
I just thought I'd throw this one out to the audience to see if any of you noticed similar sound changes with these headphones, and if any of you are in the market for an HD650, take note that you may have to change pads more frequently than the HD600 or HD580 due to changes in the sound that may not be for the better. All three of these headphones are dependent on earpad wear altering the sound, but the HD650 seems more pronounced in the changes. 

 

[Sefelt103]

I am surprised you noticed so many changes to the mid-range and high frequency in your experiments. When I replaced worn out earpads for my HD580 all I really noticed was an improvement in the bass quality, the headphone no longer lacked bass which I put down to the extra distance between my ears and the drivers. I have generally always believed that with new earpads you get the sound the headphone designer intended and as the earpads compress you begin to get less bass than the design is supposed to gve. I thought you would say the HD650 with worn earpads would sound similar to the HD580 with new earpads. Re-filling earpads is difficult and the resulting earpads never seem to attain the original shape. Comfort is obviously improved over worn earpads but if the shape has changed so might the sound. This is how Sennheiser can charge for its replacement earpads.

 

[bpandbass]

Yeah, it's really a shame they are grossly overcharging for these earpads. The problem I have with them wearing down is that the foam disks end up touching my ears, and they get irritated. With new earpads it isn't too much of a problem and I can get accustomed to the fit and the smaller openings. I found the mid bass to improve with broken in earpads on the HD580 so maybe it has to do with the progression of how much the pads wear or something along those lines.