well, I finally tried new tires both fron and back ... don't remember who recomended what, but as an experimentalist it was immediately clear that having the better set of tires on the rear was 'definitely' better than having same on front .... so, in short, replace the rears with the best tires rather than the front
cheers, guenter
ps this trial was on a 190E, improvement of having better tires on rear was 'considerable'.