thanks for the input, china_rider
I've ran some tests and it seems the best is touch as less as I can the gain in the iriver. Anyway I don't want to run too much gain in the preamp, I get a bit of distortion (even with not too loud sounds). The best results I've got so far is going something more than half way in the preamp gain and add a bit more in the iriver to can control it on the fly and stealthy.
Anyway I know the best is make my own tests and learn how to controle it, but some similar experiences will point me in the right direction before I screw up any upcoming important recording...