I recently moved from Europe to Australia and I took only the FA-003 from home. So these have been getting all my attention recently. When I go back to FA-003 after listening to HD650 and T50RP for a long time, FA-003 sound weird in high frequencies, but I kind of get used to this effect. I "forget" about it and they sound fine except for being a bit fatiguing — just a bit too much into-your-face — my ears are very sensitive.
So. While I'm waiting for my HD650 to arrive, I went to an audio shop to take a listen to some headphones. Auditioned HD650, HD700 and HD800. I immediately noticed that on HD650 that they had, the earpads were more stiff, compressed less than mine — they were newer. After I put them on, they also sounded different than mine. So I read up on worn headphone pads and found out that people don't like how worn HD650 earpads sound. I decided to order new earpads. They came here a few days ago.
Just out of sheer curiosity I took those < inner discs from the package and put them behind my FA-003 earpads. Put them on, played some rock music. I immediately noticed that lower midrange and bass were more prominent. Bass guitars were more audible — something I was kind of missing before. Also the sound is less into-your-face — a little softer, which is nice. But bass got punchier. Mind you, my FA-003 have the damping mod applied, using silverstone material.
Conclusion: these inner discs will be staying where they are. :) they must add to the loveliness of HD650 signature. You can get the inner discs separately here: http://www.custom-cable.co.uk/sennheiser-hd650-headphone-spares.html