Most people don't give the tires on their cars enough credit for how they affect the ride. Performance tires in general will result in a harsher ride due to the stiffer sidewalls and carcass. The tires are what comes into play on expansion joints and small abrupt road imperfections. I look toward Grand Touring tires as the best choice for a Benz and select for ride quality within that class. Also as tires get old and tread starts to wear down they tend to get harsh. I'm presently looking at the Conti Contac pro's as a good next tire.

http://www.caranddriver.com/article.asp?section_id=4&article_id=10252&page_number=12

http://www.tirerack.com/tires/tests/co_nextgen_grandtour.jsp

Jorg