I´ve got plans for a 15 + 6 box with Beyma 15G40 and beyma 6MI90. Where do I cross them over? How about a Second order Linkwitz- Riley @ 500Hz? Im not so excperiensed with this so please no tech talk. Also how important are the components? A 20uF cap varies bretween $90 and $1. Is there really such a big difference in sound?