Problems with designing a bass booster
You need to define an overall error tolerance in almost every active filter design.  

If less then 1dB you will probably need  a higher order Bessel LPF perhaps   4th order but staggered breakpoints.

Or you will need a 1/2 octave graphic equalizer... But warning, excessive Q in any filter can make impulse sounds loose punch. For bass the word "muddy" fits , like the kick drum impulse.  Therefore low Q filters are preferred and some amplitude error tolerance is a necessity.

Otherwise improving the subwoofer is the optimal method.