need a quick low pass filter suggestion

Have an old Orion 275SX pushing a JL 10w3.  the orion doesn't have any sort of on-board EQ to remove non-subwoofer frequencies, and i'm running this off stereo-level inputs. 

of course, the sub is trying to play full-range.  what is the best/easiest/cheapest way to cut out all the highs & mids from the sub?  I've heard about running something in-line with either the RCA's or maybe some makeshift capacitor/resistor deal in-line with the speaker wire.

anyway, if you know of what i can use, i'd appreciate it.  really don't want to buy an EQ or some other item to make it work, i know there's a way.  :)

thanks!

One of these will work.

http://store.hlabs.com/pk4/store.pl?section=9