Yes,
I have added a solid color (40 grey) to the environment, this helps the softening a bit. I also followed Romullus' tip and enlarged the area lights to 3x3 en 2x2 meter and reduced the strenght (before 2x2 and 1x1 meter). It helps a bit, but not that much.
But maybe i should increase them to extreme sizes. As i said before, i wanted to stay to real world scale lighting, but cheating a bit to get the desired result is always an option ;-)
Thanks for all your input!
PS i have added the render i made in the beginning, this one is only lit by an hdri, which i attached also. As you can see the bright spots on the hdri are very soft, this results in very smooth shadows in the render. This is the kind of softness i want to achieve, but than with the means of Coronalights instead of hdri, so i have more control over color and amount of lighting.