Vandersteen has a monitor he wanted to get flat to at least 40hz to mate with his sub's 6db/octave crossover. (On the drawing boards still?) Wilson Benesch monitors go fairly low. For less money, and less extension, there is the Infinity 2.6 active or the Revel M20. The Totems mentioned above are a good choice, also. But I agree the question is worth examining - floorstanders really can take up just as little room by the time you add stands, and have extra cabinet volume. Monitors are not for full range sound. They are for imaging and lack of cabinet vibration and/or mating to subs.
