Rega DAC users, Any info on Filter settings?


Even if you don't own the Rega DAC, does anyone know of a source of information that explains the different filter settings? The owners manual is pretty vague on this subject and I would like a little more definition if available. I have found that #4 seems to sound best on most recordings but would like to satisfy my curiousity.
Thank You.
Theo
128x128theo