How long ago did you purchase the amp? Have you called Roland to discuss the issue? I've seen that a few people online have had the same problem with this amp, so I would call Roland and see what advise they can give you with regard to settings. I'm assuming that when you were running it clean, you had it on the acoustic setting, correct?
Hiss can also be casused by bad cables as well. Try using a different cable and see what happens. When you had the GNX4 plugged in, was the hiss louder? If the GNX4 has a noise gate, turn it on and see what happens. Try using both guitars again, so when they start hissing, turn the volume down to zero. If the hissing is still there, then you know it's not the guitars. If the hiss goes away, it could simply be that the pickups (since they're high output ones) are picking up a lot of noise (which basically means they're doing their job).
keep us informed...
Jimmy
