I think what @ghdprentice is driving at--and I agree-- is that what you are noticing is not universally true for everyone who has both digital and analog front ends. I, for example, do not find myself generally playing one louder than the other. I do find that each piece of music has a volume where it sounds best, and that seems to vary by engineering/ mastering rather than genre or media.
You apparently have a system where one front end has characteristics causing the best sound to be at a higher volume than the other. I would not read into that anything inherent in the media. If you are wondering whether this suggests one front end is ultimately giving you better performance than the other, well, I think the simple answer is to ask which one you enjoy more when played at its ideal volume. You might also consider which of these volume levels more closely matches a live performance. My guess is the louder one.