I think if the system is calibrated properly, you shouldn't notice the difference in frequencies coming from the bookshelfs or the sub. Actually if it is setup properly you shouldn't.
I can see for music, yeh, but not movies.
Way I see it , is if my mains are concentrating on mid range and highs, that frees p some much amp power. I should almost never clip, specially if the lower frequencies are being produced by the 500w amp in the sub, that's what it is therefore after all.

I had the speakers finally back to my brother and a sub, I currenlty am trying to figure out what I need and want right now. and by crossing over at 80hz ( they only go to 80hz ) and a sub I found so much more detail, clarity. Like it has been said, it doesn't take much power to produce 80 - 100 hz and up, I was watching a scene in Behind enemy lines and I can here the shell casings ping as it comes out of the gun. When I switched to large some of the details like that seem to get lost.
I am going to have a reall good vacation week when I get the M80's together and switch between the two sets

Pioneer sc-1525
M80 HG Cherry