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?
It could be many things, but since you don't know, I think you should bring it to a reputable repair tech who has the equipment to diagnose it. Then you can decide if it's worth the cost of repairing.
It has worked before. I have had to stay away from the hobby for family reasons, long story. I have listened sparingly and have heard the noise only in the past few months. Because of the erratic access I haven't paid much attention. I had a local guy look at a lean problem and he found some bad solder joints. I will contact him.
I have no experience with SFL2. Does it have tubes? How long has it been since you have changed?