That's a pretty unique speaker. Is there a reason you wouldn't just use a 3rd LS50 for center?
I can give you a good example of voice matching. My best friend just bought a pair of Revel speaker from me. He has an old AR center channel to complete the front stage. When I was playing test tones to balance output levels, the LCR sweep sounded something like this:
SWIIIISSSSSSSHHH
SWOOOOOOOOSHHHH
SWIIIISSSSSSSHHH
now he didn't mind (didn't even seem to notice) but some folks would be driven crazy by that. A good tonal balance (not just SPL) across the front should translate into a more coherent final product reaching your ears.