If the previous owner decided to turn it up to 11 on a regular basis it's possible that you do have a poor example. Although you also must have better ears than I have, because I would consider the doors to be plenty solid enough to not warrant sound deadening. SEAT/VW doors are solid compared to other manufacturers! (Even if they do leak...)
I am the cars only owner. No car door is solid enough not to warrant sound deadening of some sort. Seat are no exception. You will be amazed at the difference it makes to even the OEM speakers.