The bass management allows you to set crossover points for each set of speakers--L and R mains, center, and surrounds. For example, say you have floorstanders for the main channels and small bookshelves for the rear. You can set the crossover for the larger speakers at 60Hz and 100Hz for the rears.
There was a thread recently that pointed to an article that stated bass management features actually use an average of all the speakers, rather than truly using specific crossovers. I'll see if I can find it.
Nick, I'm sorry, I don't know enough about output power vs wattage to give you an answer. In most cases, 55 watts is enough unless you have a huge room and/or plan to use hard to drive speakers. I think the main problem to look for may be dynamic peaks during movies if you have the volume at an insane level.
