Hi Markus, hi all,
after fiddling around with all the possibities I must say that the problem is the scale! For hillshading in 1:700.000 there is no appropriate DEM. Hillshading for scales bigger than 1:300.000 creates nice impressions from SRTM DEM (90m), for smaller scales than 1:1.000.000 GTOPO30 (1km) is quite right.
BUT resampling (or any other techniques mentioned) for scales around 1:700.000 leads always to very unsharp hillshades (or too much detail).
Maning's approach (r.hillshade.multi) is worth a look. I'll try it at different scales for my work! Thanks, Maning!
Anyway, I learned a lot about GRASS possibilities! 
And I learned that there are hundreds of cartographers concerned with nothing else than generalizing DEM, in Germany: Arbeitsgemeinschaft der Vermessungsverwaltungen.
Lars