Tweater hiss

That makes little sense to me.

If the his varies with the volume control, it comes from the preamp or the source.

If the hiss comes from the preamp or source, it should disappear when the preamp is unplugged from the amp. Are you sure it is completely disconnected from the system and that there are shorting plugs in amp's inputs?

How much hiss is there? Can you hear it when you are more than a foot from the tweeter? If not, it's probably OK. Otherwise, see what Bryston says.