Alot of times hissing is from the pickups, especially single coils. If you know anything about electronics try opening the back up, and re-soldering the connections at your inputs. I did this and it reduced the hissing by about 80%, then I replaced the preamp tubes. Now I would say about only 10% of the original hiss is still there.
My amp is also a few years old and I have single coil pickups.