? subsonic filter for jbl bp1200.1

I know there are a lot of threads on this one. but i had to ask myself to clear up what i need to know. appreciate all the help.

Basically my setup is for 2-15'' L7 with ported box. Amp used is the jblbp1200.1 which has no subsonic filter built in. My main question is which one do i use. i seen the fmod inline rca modules. do i use a 20hz high pass, 50hz low pass?? Confuse on those numbers.  http://www.partsexpress.com/webpage.cfm?DID=7&WebPage_ID=3&Start=4&ObjectGroup_ID=195&sm=1&so=2&search_type=56119449&desc=ASC

i am not sure, whats the best way to tell

Who built the box for these subs, and how did he port them?  In order to port an enclosure, he had to first determine what box tuning frequency he wanted to use.  Post what you got...did you buy this already set up...make it yourself...a buddy did it for you...whatever.   You're going to want to filter at about the tuning frequency for ported woofers.  It wouldn't do much good to filter at 20 Hz while the subs are tearing themselves apart at, say, 39 Hz.

Also, post what description of problems you are having......that you are looking for a low frequency filter.

its a xe designs box. http://www.sjgreatdeals.com/xepl7215.html got it from the wholesale company petra.com. there is the link of exactly what it is. i'm not sure on the dimensions of the box to be right. i will measure the box and port size to be sure tomorrow. i don't have a problem, just noticed that kicker highly recommends a subsonic filter so that the subs last longer.

ok, decided to go ahead and get the dimensions.

38''w X 17.75" d X 17.75" H

port size is 1.5in X 16in

let me know if you need anything else. thanks again