I have 3 pairs of headphones, all are from beyerdynamic, more specifically: DT770 Pro 250ohm, DT990 Pro 250ohm and DT 1770 Pro X, it may be irrelevant in the context, but I use all 3 of these in the headphone sockets, headphone knob 1 I can turn from 0 to 100 without any hiss sound, but headphone knob 2 and 3 I can only turn up to about 30% before it starts to hiss, does anyone know about this problem? or how to solve it.