Hoffman's Iron Law: Low, small, sensitive. Pick any two, but you can't have all three. In a car you might get 20Hz from a 1 cu ft sealed box with sufficient sensitivity to give adequate output, thanks to cabin gain, but that's not going to happen in even a small room. In a large room you need to either quadruple the box size or give up an octave of response. As for not wanting to use multiple subs, that's the room's decision. Depending on the room and the size of the LP you might get away with one sub, but you also may find it sounds different throughout the room. The only cure for that is multiple subs. However, that does offer the potential to use smaller subs to achieve the desired output level and LF extension.