Predrag,
We have a CL5 and it also has a hiss. We too have updated everything and played with Rio boxes, and everything else we could think of. Fortunately for us, the hiss is only on monitor mix 1 output. We initially blamed the monitor, JBL VP series. But we switched and swapped out everything and it is still there.
It could be Yamaha has a buggy problem here and there. Please post if you find anything out. We've contacted Yamaha, but so far, no response. Our rep is trying to help us, but that is it.
scott