I think that there has been a steady decline in the comfort of Mercedes seats, at least in the 124 models. My 99E300D seat is hard as a board and in no way compares to the seats in my 87300TDT.
In the 99 there is also an almost completely unpadded bar at the bottom of the seatback that hits me right on the tail bone! Unbelievable. To me the second most comfortable seat in the W124 models is the MB-tex seat that I had in my now sold 95 E300D. Enough padding, enough support, plenty of adjustment, with the bonus of being easily cleanable. The leather in the '99 is apalling--looks more like plastic than plastic does. All that being said, I still like the car. Now if I could shoe horn a seat from an '87 300SDL in to it... As far as what you can do to make your seat more comfortable, I don't know, other than to find a car with the Tex interior and see if you like it better.
Steve