Can you get flat response with Linkwitz-Riley cross-over? Many pictures display that the Linkwitz-Riley has $-6\textrm{ dB}$ dips at the cross-over frequencies.



Are these fixed or can these made flat?
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.