I had the Porta Pro's for a few weeks. I enjoyed the warm signature. The only problem I had with them when on the headband was that the bass could get congested/bloated, and the treble seemed to fall off quickly. I tried them on the ksc75 earclips, and liked that quite a bit (I suppose that would essentially be the same as the ksc35). Even though the bass still could get bloated at times, it was a nice sound, although I didn't find is super-detailed. I ended up getting the Senn PX100-II which has a similar signature (the bass is still there, but better controlled and more details). My go-to phone before getting the S500 though was my ksc75 on headband - they sound great to me - very nice, detailed and forward mids.
The S500 is a different beast altogether. Strong bass (and sub-bass), but very well controlled. As are the mids and the highs - everything is well controlled with lots of detail (and this is with maybe only 50 hours of burn in). The thing that strikes me about them compared to my ksc75's or px100-ii's is (a) the sub-bass and (b) a certain depth to the sound - kind of a 3D-like quality. I liken my ksc75's to watching a great high def HDTV in 2D, and the S500 to watching it in 3D. Kinda goofy analogy, but heh.
I am getting the ATH leathers to try out for a bit more comfort and because they might make the mids a bit more forward. But no, you don't need different pads - the stock pads sound great and are comfortable enough.
Edit: I did get the ATH leathers (pleathers, whatevers!) onto the S500. I have only listened to this combo for half a day now, but based on the change in sound, I have to revise my above statement to the following: The S500 + ATH leathers is amazing. If you think that maybe you could get a bit more out of your S500's, or that the bass is a bit much, or that you wish the mids were a bit more forward, then just do it: get the ATH leathers.