My C36 project came with the 075 driver and no lf driver, but included an adapter ring for the 12" D123 driver...which I installed. This left a third, covered hole in the baffle for a third driver. I left it as is, thus no vent anywhere in the cabinet. Is this correct? I see Viscounts and such with rectangular vent hole or an open hole where a third driver could be installed. Please enlighten me! I hear cabinets need "tuning"