This is just a thought, but wouldn't it be resolved by just going to iTunes, selecting all under Edit, right-click on selected tracks, select Get Info, then answer yes to Edit information for multiple items, then just use the volume slider to adjust the audio db output for all of the tracks in your iTunes library. Maybe I'm wrong here, but I'm thinking that when you then sync your aTV from that iTunes that the db output for all those tracks will carry over.
Experts? Am I onto something here or just peeling a waxed banana instead?
(Of course the downside to this would be that this same decrease in volume level would carry over to everything else that syncs to the iTunes.)