PTS,
I am still wiring my GR Research/Rythmik DIY sub project, so can't answer the question about the resulting sound. I am using an in-line filter to roll off my amps at about 48Hz just to my main speakers. My main speakers also go down to 30Hz. The filter was built by Danny at GR Research specifically for my application. Think about contacting Danny, because he knows his stuff.
Kenny