Very faint hiss from tweeter

It normal in most cases . Turn the Amp on without the preamp.   Still there?  Normal to some degree, depending on speaker.   Try to make sure interconnects and speaker wires are apart and also not intertwined with AC or ethernet. 

Still exist?  Try different tubes in the Schiit

All of my systems have had some sort of very faint hiss when no music is playing, and I put my ear right up to the tweeter. It’s a good way to confirm that the tweeters are working, and should pose zero issue when listening to music.  No problem if it's not loud enough to be heard across the room, or when music is playing.

"if I put my ear very close to the tweeter on either speaker, there is a very faint hiss sound."

Avoid future audio neurosis- don't put your ear up to the tweeter.

Best long term solution for A.N- stay away from audio forums.

IME, a system with tube electronics likely produce a faint hiss; which is normal. As long as the hiss noise doesn’t intrude or increases as you raise the volume or audible from your listening chair, you got nothing to worry. If it bothers you, then be ready to spend some money and considerable efforts on gear swapping journey.

Thanks for all the feedback!  Glad to hear I don't have a problem, perhaps aside from a need to do some cable management.