I lived with my HD 201s for a couple years dealing with the lack of bass until I finally decided to upgrade, mainly for more bass and better sound quality. I bought and extensively used the HD 438s, which sounded good and I thought I would keep them even though they didn't give the bass I really wanted.
Then I tried the HD 438s pads on my HD 201. I was shocked; instant increase in bass and horizontal sound stage. I bought a pair of the HD 438 earpads from Sennheiser over the phone and did a direct comparison between the HD 201s and the HD 438s with the same ear pads. Once again, I was shocked by what I heard. The HD 438s no longer sounded like they were worth $90, but more like $50 at most. After a few songs I decided the HD 201s held so much value for $20, and then add the fact I could get $90 back by returning the HD 438s and keep the HD 201s with the new pads. I recommend that with a brand new pair of HD 201s you wear the headphones with the stock pads as they should sound decent with the still tight head band clamp. Over time, as the headband loses its force and thus perceived bass decreases, you will find more of a need for the better pads, which should immediately make the phones fit more tightly.
The pads can be found here: http://www.sennheiserusa.com/serviceandsupport_part-finder
The ear pads are found under the product model "HD 438". The pads will come within a week. They go for a bit over $15 including tax and shipping, so the investment is definitely worth it. If you have any Sennheiser around ear phone with the pleather pads and are looking for more bass, try these out!
Edited by TrantaLocked - 12/30/14 at 9:02pm