There's no difference in the low frequency coupling with the floor with vertical versus horizontal placement. The wavelengths of the coupled frequencies are far too long, ten feet and longer, for the cab/driver orientation to make any difference. What is different is the audibility of the shorter wavelengths of the mids and highs. You can't hear them as well when the source is down low, and that makes the low end seem more prominent, whereas in reality it isn't. This a classic example of where "I know what I'm hearing" is actually a case of no, you don't.
I was thinking that if you position not one, but two or multiple woofers near the floor (horizontal), it could be different from the situation that there is only one woofer so close to the coupling (and resonating surface). Every time I did a problematic gig, like hollow stages, I did notice a great difference when stacking cabs in different ways, and this even on-axis and at ear level. I do not know for sure about this, I'm not a loudspeaker designer like you, I just observed carefully an empirical (for me) phenomenon. But I can tell one thing for sure. I use two 2x10 cabs that I stack in one way or another. Because of the shape and measures of the cabs itself, placing them vertical or horizontal makes the whole combo almost as TALL, the difference is quite a few inches (or cm, if you like european). But the tone of the stack is dramatically, I mean dramatically different, in the way that I have described before. It obviously also depends on other factors, other than stacking mode (room, outdoor etc, but the trend is still there).
Therefore, you have to be patient and polite, english is a foreign language for me, and sometimes I do not understand well, maybe I'm confusing things and concepts...if the wavelenghts are so ample that, I say, 20-30 inches do not matter, obviously 8 inches even less, ok? (8 inches represents the exact difference in HEIGHT between horizontal and vertical, and the complete stack is 46 inches tall in vertical "mode", but I use a cart under it, so in reality is 48-49).
You can add to the case that my two cabs have a tweeter each, so there is always a source of highs at the very top of the system. Sure if you are in front and at ear level with a conventional 4x10 you WILL hear better those directional mids and highs, whereas the bass is omnidirectional (I'm a medical doctor, psychoacoustics matter when talking about hearing)
If I can learn something, I will listen to you very very gladly. Just I doubt that the difference in the tone, so evident that even a deaf could hear it, even just in front of the system, is caused only by a taller setup (8 inches, but this does not apply if you sit down on a chair in front of the cabs).
...although having a 15 pounding you in the back of the head is a very cool thing.
* the big-stack* 
