The onwalls are a bit more efficient than the Maggie floorstanders, but I would try to get away from using a receiver's amp section to power them. Does that receiver have pre-outs? If so, you can use the same receiver for the processing part, but add an outboard amplifier to power the speakers. As for amp suggestions, I'm sure we can all come up with some good ideas, but you may want to provide a budget for us to work with.
EDIT: I just looked up that model and it does have pre-outs. So all you need to do is add a multi-channel amp to it and you're good to go. Are you running 5.1, 6.1, or 7.1?