a hiss and a humm can mean conpletely different things, so you'll have to decide that one for yourself. If its a humm then possibly a ground loop as said above but if its a hiss it could be all sorts of things. On an older amp i might suspect the volume control, unless it seems to be exactly the same when you move the knob. It may also just be the amps own hiss. Usually you would notice it on cheap amps alot more then something decent so its hard to say. I wouldnt worry about it though because it doesnt sound like it interferes with your music. I get something a bit similar with different tubes in my preamp, just something i live with
