Sennheiser HD 580/600/650/660S sound signature depends heavily on its earpads. Changing pads will significantly affect timbre, and probably not in a good way.
To emphasize how significant the change is - A year old Senn that was used daily does not sound the same as a brand new unit. This is due to earpad degradation. If you use your Senn as a reference headphone, it is recommended to change OEM pads every year or so.
Aftermarket earpads for HD 580/600/650/660S (such as these or Dekoni) are fun experiments, but please keep in mind that you are deviating from the manufacturer's sound signature.