[JorgenMan]

I'd be very impressed if you could come up with a design that does 14 Hz at a level appropriate for 3500 cubic feet for $500, especially in an 85 liter box. The sealed 12" Rythmik has good frequency response, but might not be enough for such a large room - it looks about borderline according to Rythmik's reccommendations. The vented will not get good response down to 14 Hz. If you can live with reduced output levels, the 12" sealed Rhythmik might be okay, but otherwise, I think you'll have to change your requirements a bit.