I have been crunching numbers and am thinking of just buying PR's. If I go and get the PR's how much weight do I have to add to tune them to 18.9 hz? and the box volume needs to be 30l correct? so build an enclosure that is 33-34L right becuz the driver is ruffly 2L and the PR is probably close to the same. I will still need that sallen key 2nd order filter correct ? So if you could let me know how to build it or what exactly it will do i will be greatly! appreciative. Thanks