Hi ABM users: I've just had a similar problem to Dagrev occur: when there is nothing plugged in to the amp it is silent with the master volume up; plug anything in and it hisses, even when the input gain is right down (and master up). I've tried different cables, basses and the problem remains; and these same cables and basses are fine into another amp. The hiss is right at the 2.6kH frequency, so it can be pretty much eliminated by taking that eq fader down to the mid point, but I need to get it fixed. The amp was serviced three months ago when it had a new preamp tube and a thorough pot clean. Any insight into what this might be? I can easily take it back to the tech who serviced it, but would welcome any thoughts first.