One or two subwoofers?

Adding subwoofer to two-channel system. Main speakers are a MTM two-way design flat to about 45hz (Northcreek Catamounts). Hoping to get down to 25 to 30hz with subwoofer. Looking for fast, tight response to match main speakers. Subwoofer will be integrated through Tact room correction pre-amp with sub-outs for up to two subwoofers. Room is medium, about 10' x 17'. Have read elsewhere that two subwoofers are best. Is this true and, if so, why? I thought low bass was non-directional, so no loss of stereo effect by using one. I'm willing to crowd the room with two subwoofers if necessary for best effect, but would prefer just one. Your thoughts? Thanks in advance.