The spread is so modest between the mains, relative to the distance to the LP, that I would undoubtedly get rid of the center speaker altogether. Those mains should image well enough as phantom, and no more worries of acoustic mismatch either. Besides, the thing says Bose on it.
Just turn center speaker off in the processor, and this channel is rerouted to mains. You can possibly suffer a bit more compression, but maybe you don't even listen that loudly. I bet it will sound better. I know that's what I would do, no doubt about it.