Bill, I believe you to be in error with concern to how room size effects a given frequency.
The room will either amplify or mute a given freq., nothing more. The sonic wave of a 15" sub can be easily felt at 20Hz in any typical listening room. As I previously stated, high spl requires high wpc. So if one is going to purchase a hi power amplifier, why then use small drivers? The answer is simple, SPL. Larger drivers get this same SPL using far less power, given that the drivers are of equal efficientcy, however, larger divers (using the previous premise) can also reproduce lower registers of sonics with much greater ease and finese.
Why incorporate another "box" into the mix (your usage of a "sub"), why not simply incorporate the driver where it belongs- in the Speaker cab along with the mid and hi-freq. drivers. Unless of course you belive that bass is not sterophonic, or that time alignment means nothing- I wonder?
The key to any good speaker IMHO is much the same as your thinking, however, if the speaker is really good- one doesnt need a sub do they?
Unless of course were speaking in terms of electrostatics, and that is a whole new ballgame.
There are many companies that make very good ful range BIG speakers, however, only a few do it correctly. Wilson, Martin Logan, Legacy, older B&W, older JBL, Magnepan, just to name a few.
No offence, but size does matter in this case.