[LAD] Linkwitz-Riley (LR-4) digital crossover filter dsp

After reading extensively on digital filters used as crossovers, I just finished implementing a digital LR-4 crossover filter using 2 chained butterworths 2 pole q=1/sqrt(2). I used the bilinear transform method to calculate the coefficients of the butterworth filter and chained 2 of them together to make the Linkwitz-Riley. I wrote a simple calf plugin using this dsp. The frequency response curve of the output shows accurate 24dB/octave rolloff with white noise fed into the filter, however the highpass filter curve shows considerable noise at low frequencies (under 40Hz). Should I be using a different algorithm? I read somewhere that "biquad" is a good method, but I have no idea how to do it. Regards, Damien