The 805 and BeoLab 3 are two quite different speakers - both fantastic in their own different way, mainly as a result of the Acoustic Lens (which makes the BeoLab 3 extremely different in it's character).
B&W speakers tend to be extremely picky about the amplifiers you power them with, and I'd say Marantz is a no-no (I've had experience of this exact combination lately). Look at Classé, Bryston or at the least, Rotel amplifiers. Once you've found a sound which works for you, integrating the amplifier shouldn't be an issue.
BeoLab 3s are the simplest solution to go down in terms of plug-in-and-go, but as above, they are really quite different in the way they sound...