Subsonic Filters

i have always had Rockford fosgate amps an am very familar with there ability to make subsonic filters with boost if desired..  do any of you have any experience with making these without the use of an card in the RF amps (old Style)  i am using a ported enclosure and would like to maximise its spl and durability..  i have a newer kicker amp and some older rf amps and am deciding which to use... 

thanks

ex.. i have a 1.25 cf box with idq dvc tuned to 28hz

most subsonic filters, I believe, are just a narrow band, bandpass style of filters, tuned to to cover say from 5hz - 30hz.  I am not sure about "maximizing" the SPL in this case, but I can tell you how to build a filter to cover these ranges.  I would probably use a butterworth filter for this scenero.  They have on this forum a calculater to make simple band pass filters from 1st - 3rd order, but I am not sure exactly how accurate those are without simulating them in Matlab or something to see what type of responses I can get out of them.  It is best to build to to be an "active crossover" vs a passive one, so if you could put your crossover network prior to the amp that would be optimum.  However, I am not sure what your restrictions are, so let me know, and I will see if I cannot help some.  :)

Craig