I did some more testing and my puzzle seems to become a bit clearer.
I made the differences a bit bigger and printed 4 step wedges, dotgain 10% and dotgain 30%, each of them with the profile assigned and converted to that profile. Now I sees indeed differences between prints the converted files. By no means as huge as I expected based on what I saw on my monitor, but they are different. Now I know where to look I see the differences also in the less extreme cases. It learns me as well that I need really good light to inspect my prints I don't see differences in het prints of the assigned profile files, which is a confirmation that QTR does not use assigned profiles (as I read somewhere).
So this part of the mechanism becomes a bit clearer. Remains my question:
Why should I before printing convert to different profiles for matte and photo papers? Are these differences not already accounted for in the QTR curves?
Or are the matte and photo profiles only meant for soft-proofing?
Thanks
Joost