I recently bought a used DB659 and the tone sounds great but I've noticed a slight hiss. It goes away when I turn the first stage volume down abut when I turn the master down, it increase a bit in volume. The ground lift on the unit doesn't help... I have hum frees on my rack gear as well. has anyone had a similar problem that you've solved before I de-rack it an hike it in to my tech? Could it possibly be a microphonic tube about to go up? Thanks in advance.