You should be able to get away with using just 1 channel on your amp for both speakers.Sorry to disagree, but B&W states the impedance of the LM1 as 8 ohms with a minimum of 3 ohms. Using a pair on a single channel will result in a 4 ohm impedance with a minimum of 1.5 ohms. The recommended minimum impedance for the SR-6004 is 6 ohms. It just seems too risky, mainly because of the severe drop to 1.5 ohms. I would talk to Marantz tech support before attempting to use a pair of LM1s on the center channel.
I would definitely use a single LM1 for the center, as this will give you a perfect timbre match throughout the entire room.