Rcprince is correct. I recently posted a thread on this subject. I have a Marantz DV-12S1 that allows the user to toggle between a 24/96 digital signal output and one that has been 'down-converted', depending on what one's processor or DAC can handle. Unfortunately, the manual also clearly states that many DVD-A discs may restrict the player's ability to output 24/96, regardless of setting, due to piracy and copyright issues. My player's on screen menus will display the signal being output, whether it is 'down-converted', 44, 48, or 96.
In my limited experience with DVD-A's, I have yet to find a disc that is not locked. Unless I can find a way to mod my player to circumvent this glitch, I see no reason to purchase an external DAC.