Can't find the thread, but I remember seeing someone mention they could "up" the voltage of a HU by adding an external optical output? Not sure what this is or anything, but is it possible to upgrade the voltage and/or is the sound quality that big of a "difference," or a noticeable change between 2.0V and 5.0V? I'm thinking about picking up the DDX-7019 or DDX-8019, but would rather the 7019 for $200 less!