Center Channel Speaker Question

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 LM1’s 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.

Yes, a series connection will be safe for the amp, however the resulting 16 ohm impedance load will reduce the center channel amp capability from its rated 110 watts to about half or 55 watts. This will then be split between the two speakers. Sound from each LM1 will likely sound thin and anemic, and have very little dynamic range. It makes more sense to run a single LM1 with 110 watts than 2 with less than 30 watts each. A series connection will just reduce the overall performance.

Now I am not against using 2 center speakers, actually if it was safe to do a parallel connection, I would say go for it, however I do not agree with,

You can try it that way but your amp may strain a little.

Initially it might work OK, but long term is likely to cause premature failure. Again, talk to Marantz tech support.