I found out something interesting. The hiss is only noticeable when the pedal is in pre/post configuration. I mean when inputs and outputs 1 and 2 are used. Otherwise the hiss is gone. If I put h9 in front of my amp in normal configuration (using only input 1 and output 1) I am not able to hear the hiss using all my amp channels. 

I realized that in this pre/post configuration the hiss is gone when I swicth off the effect loop