Ok. Increasing the volume to 100% in iTunes does not help. It simply allows you to reach the maximum volume "earlier" on the volume bar. It seems ti simply makes the volume increase non linear with the volume bar. That is strange, to me. I do not get the purpose of such a feature. Max volume is stil the same. Poor quality is unchanged.

So, I have only two options left, I believe:

1. Installing some US firmware or software in the iPod. I don't know how. Maybe, as you suggest Mark, trying to use some US-based iTune account.
2. Apply the change to the iPod sw using the Hex editor. Very risky as even simple software upgrades may make this change very unsafe.

Anyway, I am still hoping that the volume increase will also bring aome quality increase, assuming the volume cap has some side effect on sound quality. If not, this still remains a very low quality audio device in that regard.