Pass 150.5 hissing and popping

Hi all, I'm hoping you can help me with a problem. I've understood that the amp should stay on all the time. However, when left on for a few days, my speakers (Monitor Audio Gold GX100) start to pop, crackle, and hiss! 

If I turn the amp off for a but, the hissing or fedback disappear, the imoes away and the sound is crystal clear again.   All the bindings are tight and well fitting.

What's the problem???