FWIW,
I had floorstanders (Paradigm Studio 100 V 3) that sounded much better with a sub-bass unit (Rel Storm 3), but it was vital to have the Rel meticulously adjusted by a trained salesperson. I now have Intuitive Design Summit Loudspeakers which would technically be considered a monitor, but which are on heavy, sand-filled stands, and the stands are on 75 lb granite surface plates, so the footprint and overall dimensions of this assembly are actually larger than the Paradigms'. I have kept the Rel, but the crossover is set quite low, at 28 hertz. The Paradigms had very good bass for a 2300 dollar speaker. The Summits have even better bass, I think.
However, several other variables in the system have changed, including the replacement of the Rotel RC995 preamp with an Odyssey Tempest Extreme and the addition of a Dodson 217 D Mark II DAC with upgraded 218-like software. Since the front end components are now better, the above comparison is not completely fair.
Importantly, I have not perceived any significant problems in terms of being able to localize the bass to the Rel, which sits in the corner. That has been the case with both the Paradigms and the Summits. It may be wise (or at least fun) to read the "Velodyne Digital Drive Series subwoofer in stereo" thread about this, however, where some of our fellow members get into a pretty amusing argument over the merits of one versus two subwoofers.