When using the 5.1 outputs of the player, you are using the player's processor. So, you need to set the speaker levels in the dvd player's setup menus too. Giving the subwoofer a little extra adjustment in the levels setting doesn't hurt either. I have the crossover set to 120 on my 563a, and it works fine.
Also, in the setup menu settings of my 563a, there is an area called "Options", then "DVD Playback Mode". The choices for this is "DVD Audio" or "DVD Video". Make sure DVD Audio is checked (if your 578 has same menus), instead of DVD Video. I think some people's first reaction is to check Video, but that isn't right, for this menu. If you have Video checked there, change it, and see if that doesn't make a difference.
Unless you happened to get hold of a bad unit, I think the problem is somewhere in the setup. My 563a gets plenty of bass and your 578 should also.........
Good luck!