Can you get flat response with Linkwitz-Riley cross-over?
Linkwitz Riley filters always have  -6 dB at the cut off frequency. The whole idea of a cross over is to split the signal into multiple bands. The advantage of a Linkwitz Riley is that it adds to be flat (at least in amplitude). So each the highpass and the low pass have -6 dB at the crossover, but the sum of both is flat.