Dan- I wouldn't worry about it at all, most all tube amps hum when music is not coming through them and LP's forget it even a great Turntable and new LP will hiss. Which isn't a bad thing just the way it is. I realize you don't have an analog front end or a tube amp but trying to tell you that hiss isn't a big deal at all, in particular when you have to be that close to the drivers to hear it, if you could hear it over what ever your listening to I would be a little more concerned but you should be fine.
Cheers! Tim
Cheers! Tim