Bugger. Hopefully its a fixable thing. Sounds like you've covered the various testable stuff. Have you tried just taking the line outs and sticking them stright into a line input on a mixing desk or something? Then whack a pair of headphones somewhere and use your ears to tell you if the Hiss is coming form the RP, I'd do it this way instead of using the phones on the RP, then you are testing the signal that would go into the amp. Don't want to tell you what to do, (you probably already got it coverd) but one thing I would definatly check is the level going into your amp/s. If you are going into your amp via the RP outputs, try plugging the guitar stright into the amp and see what sort of volume level you get with the amp set to say 1/3, then put the RP back in line with it and see if its about the same. If so then its not levels, if not adjust the RP so that all the Present levels are around 40ish and adjust the master volume of the RP to level match your guitar without the RP in the guitar to amp chain. Its just that Hiss is usually the noise floor of a device, so it could be that the signal isn't loud enough somwhere and the noise from boosting it at the wrong stage is causeing Hiss, or its too loud and the noise floor is way higher in level than normal. Or it could just be something is kafuckt =) I guess. Cheers.