It is possible that the problem could be unrelated to the empeg. Check your amplifier and its wiring.
In fact, the next time the hiss happens, leave everything running but unplug the RCA cables from the amp, at the amp. This will leave the amp powered up (the amp remote line is still "live"), but will disconnect the empeg from the loop. Then if the hiss is still there, it's the amp.
Although this is a different problem, it illustrates the sort of thing that can go wrong with an amp: One thing that happens on one of my amps is that the right front speaker sometimes gets crackly. It sounds like an EQ or an MP3 problem when you hear it. But the truth is that the gain adjustment pot on that amp channel is just a little iffy, and I just need to turn the knob to sweep the contacts a bit. I have to do this procedure about once every two months to keep the speaker from crackling.