Humming and static noise, Plinius SA 100 MkIII

Hello. I am currently using a pair of Plinius SA 100 MkIII's as mono blocks to power my B&W 802D's. First the amps have a slight hum when on and sometimes they are very quiet. More importantly, when I turn the volume up all the way without a signal from source, there is a very noticable high frequency hiss like radio signal distortation at the speakers. I have tried many things including changing preamps, cables, cords, etc but nothing worked. These amps generate 400 watts as mono so with the volume turned up all the way the noise may not be abnormal.

I would appreciate if any one with similar issue has an answer for me. I love the amps but the noise level is a bit too high. Note: in normal listening volume the noise is very low and not noticable at all only at vey high levels the noise is very noticable. Thank you.