First off, accept the fact that the speakers aren't generating the hiss.
It originates in one of the upstream components and they simply reproduce it. The only real way to pinpoint it's source is to isolate it to whichever component generates it.
Does it do it for any source or only one source? If only one source, then the cause is obvious.
Does is do it when only the power amp is on and not the preamp? Then I'd be looking at the power amp.
It may come down to actually removing the components from the circuit one at a time. HEck, it's even possible it could be a bad interconect but I kinda doubt it.
etc... etc...
IMNSHO, I'd suspect the preamp but I've been wrong before.