Hiss noise from my SFL2 preamp

I have had a hiss level from my Sonic Frontiers SFL2 for a bit. It is constant, loud enough to hear across the room and does not change in volume with the volume pot. It goes away if I mute the preamp. I have all new tubes, changed them around and no help. Balanced or single ended mode both. The phase switch when engaged is much noisier and more grunge sounding. Any ideas?