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

[Damien Zammit]

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