You are absolutely right. I have set up systems with LS3 5As on stands with subwoofers and blind folded you would think you were listening to much larger speakers. They also image better than many larger speakers and there is less enclosure coloration. You have to run the subs up to 100 Hz to get the best effect which requires very steep cutoffs.
There is one issue I think you need to think about. The ability to move "fast" determines a driver's high frequency limit. Most subwoofer drivers run easily up to 500 even 1000 Hz. 100 Hz is never a problem. The larger the driver the slower (shorter excursion) it has to move to produce the same frequency at identical volumes which is why larger drivers have less distortion and frequently better transient response, just the opposite of what many people think. The same is true for multiple drivers as it is really a surface area thing. To make really accurate and powerful low bass IMHO you need at least two 15" or four 12" drivers in most average size rooms. I use eight 12" drivers. Above 15" the cone becomes more difficult to control. I have seen strobe films of 18" drivers moving in some wild ways, anything but pistonic. I would use Eight 15" drivers but the size of enclosure required would not work in my room.