Originally Posted by JerryP

Hmmm boy it's hard to say what's going on with your amp without hearing it myself. The hiss is normal for a 5150. The lead channel especially when you get the pregain up past 4 or 5. When you plugged into the loop return you were bypassing the preamp and going directly into the power amp, that's why it was clean but it should have been fairly loud. If you can find some other tubes I'd try them, preamp and power and see what that does. Maybe take some out of a friends amp just to test your amp? Other than that it might be time to see a tech.
Jerry