I thinks the ATH WS55 pads should be the best in terms of sound and comfort if you just want to slightly deviate from the S500s stock sound. The big over ears HM5 pads change their sound too much, leans out the bass, so you will have to do other mods to get the bass back.
I would just get the WS55 pads direct from ATH USA for ~$10 shipped. I have them on my ES700s and they are great in terms of comfort and sound, never tried it on my S500s cuz don't want to mess up my godly HM5 modded set. :)