[Cpt Rick]

Alright, I'll be the one to ask the question that we all know needn't be asked, but sometimes can solve the problem quickly:

Is everything grounded properly to a common ground?

The next question I would ask is does your mixer have an effects loop, and if so, why aren't you using it?

But, what I would do is take all the pieces apart, start with just mixer, amp and speakers and then add everything in one at a time until you find the source of the hiss. I have hiss on my system, but I know in fact that it comes from the mixers EQ section. So, I know how to deal with it.