Thank you for your tips!

After having tested a while I found following (partial) solution:

> at least when using the Pandasoft Direct X Exporter the option "Optimize mesh" has to be set to "Normal" or "Optimize", otherwise the faces of the mesh will appear faceted (no smoothing at all). No doing that was my original fault.

> adjoined faces appear faceted if their congruent faces are NOT welded in 3ds max
> adjoined faces appear smoothed if their congruent faces are welded in 3ds max
> parts of a mesh which have different textures have to be seperate models in 3ds max

Still there is one problem:
The specular color shows harsh transitions exactly where the seams of the UV-coordinates are located. The only workaround is to use just flat projections for the textures, which is very limiting.
The above mentioned problem doesn't appear when using the old max2mdl exporter with the same model.

Does anybody have an idea how to overcome this problem?

BTW: in parallel to 3DGS I use Quest 3D. When importing (the same) X-files to this applikation no such problem appears at all. Even there is no necessity to prepeare the 3ds max models in any of the above mentioned ways at all. Smoothing groups from 3ds max stay as smoothing groups in Q3D, several textures can be used in one mesh and will be transfered ...

So apparently the problems are caused by the X-iport filter of 3DGS, not by limitations of the x-file format.