I'm now thinking that you have some kind of ground loop. You say that the hiss goes away when you kill the effects loop, so maybe this is where it is coming from.

Are you able to lift the grounds from the cables going to first input 2, then output 2, and see if this makes a difference ?

It's very likely that this has a hardware cause specific to your setup – Pre/post wiring can be difficult, and there is no one-size-fits-all solution.