Ragobro, I have found my '91 300SE to be the most sensitive car to tire balance I have EVER seen. We just went through 3 attempts to balance a new set last month; the tech finally got them right by setting the Hunter machine on "manual" and working them to within less than 1/4 ounce. This happens every time I buy new tires, and I've had her for 240k miles. Once right, they seem to do fine for many miles. The classic symptom is vibration right at 60 MPH, going away at slower or faster speed. Condition of wheel bearings, idler arm, damper, ball joint, bushings, etc; has seemed to make little difference over the years. I've rebuilt everything a few times, and still get balance problems with every new set of tires.
The clunk you hear is quite possibly the guide rod (caster rod) bearing. This is the broomstick-size rod that runs straight back from the bottom of the shock absorber/control arm to the body. The bearing is mounted on the rear at the body attachment subframe. It can be verified by lightly applying the brakes when going over the bump - if the clunk goes away, it's probably the bearing. Do a search, you'll find lots of photos, etc.

DG