You're on the right track, it's coming from the pre. Klipsch sensitivity isn't helping matters.
It could be a simple gain staging issue. Does the Halo have gain controls, and where are they set at? If they are maxed or in the top half of their adjustment, reduce them. You'll have to push a higher signal from the pre, but that will help S/N, hopefully burying the noise. Report back if this works, I'm prone to wild guesses.
edit: I see the Halo does not have gain controls. So much for that idea.
Make sure the grounding switch on the back of the amp is in the proper setting for balanced inputs. Check your cables and connections. Not sure what else to suggest off the top of my head.