Rear springs too hard?

[94E300TD]

Hi

My 94 wagon is quite bumpy and stiff in the rear, when the car is empty.
The car feels uncomfortable
The rear springs are of the reinforced type.

When there is some weight (150 kg +) in the luggage compartment, the ride is much, much better.
The car feels more "Mercedes-like" and comfortable

The self-levelling suspension works fine, and the accumulators are almost brand new.
Is there any reason to believe that this behaviour can be caused by anything else than the reinforced rear springs?

Thanks in advance for advices...

Regards

1994 E 300 TD