corner frequency (f=1/(2*pi*R*C)
For low-pass, R = R8 || (R5+R6) = 5K, and C = 10nF.For high-pass, R = R8+R5+R6 = 20K, and C = 1uF,
Your gain in the "mid-band" = R6 / (R8+R5+R6) = 0.1, so you're losing most of yoursignal right there.
R8 = 2.2k (although my target value was 1.4k.... I also used a 1K value in place as well)C18 = 10nFC21 = 1uFR5 = 8kR6 = 2k