Thanks for sharing.
If someone wants a 24db slope without a silly price Sublime acoustics looks like a good option.
Hsu has custom high pass filters
I've been constantly recommending the use of a high pass filter, along with plugging main speakers when ported, as the best way to integrate subs to main speakers but never really thought about it much besides using a single series cap.
Today I stumbled across this nifty and cost effective product while considering some new subs. The advantage of the active high pass is you get 12 db/octave instead of just 6 db/octave. This can help remove existing, bad speaker/room interactions and improve the ability of a sub with EQ to work.
Hsu will custom make a high pass filter with 2 crossover points. For instance, you could do 40Hz and 80 Hz.