I put an MXR Noise Clamp in my rig to cure exactly the same problem, but I suspect any 4-cable noise gate will work. If you're familiar with how to use one of those units to kill noise from pedals, just imagine your guitar's preamp as a "pedal" and insert the gate into the effects loop. It really took the hiss out of my gain channel, with no noticeable tone loss. If you like everything else about your amp, it's definitely the way to go.
The MXR is the cheapest unit around that I know of, it lacks the tight cutoff response of some of the more expensive units but it works fine for my needs. If you're playing really chunky metal, you might want one of the ISP Decimators, but if you just need to kill the noise when you're not playing, like in quiet passages or between songs, the MXR or Boss gates should work fine. My only word of warning, it may cut off your amp reverb when engaged, and you have to put your delay pedals AFTER the gate or they'll get cut off too.