Macrojack: Naturally your speaker cabs do resonante -- all speaker cabs resonate. I'm surprised that you and Line say you can't feel any vibrations with your hand placed on the cabinet. This is true with any surface, at reasonably lifelike volumes, playing any music? As for rocking, most speakers won't do this to any detectable degree if well designed and anchored.
Even if they don't obviously vibrate a great amount, the large areas of the cabinet surfaces involved in making a 4 ft. high speaker means there will be sonically significant unwanted contributions from the cabinet. The effect is potentially worst with speaker constucted from flat panels (as most still are) -- speakers with curved cabinet surfaces have a theoretical leg up here, especially if we're also trying to keep weight down to sane levels.
No matter what you do with the bass, cabinet talk will exist if there's a cabinet involved, but obviously speakers that roll out on the bottom sooner than others will have lesss of a problem to deal with, and this I assume is a factor with your speakers. BTW, I agree that 70 lbs, which is what my own Thiels weigh (with their 1" panels, 2" baffle, and 5 internal cross-braces), is at most mid-weight for a 4 ft. floorstander.
I do have to admit to a lack of basic understanding in a technical aspect of what you're alluding to here though. John Atkinson has also commented in a few speaker reviews that higher sensivity should translate into less cabinet talk. I think I understand why this would be true with horns, because they work by concentrating the sound pressure in the direction of the listener, thus reducing the overall driver excursion needed to achieve equivalent perceived volume levels. What I don't understand is why this should hold true for direct-radiators like yours, because the amount of air that's required to be moved to achieve any given volume level is the same, no matter what the efficiency of this type of speaker. In other words, I understand that the required amount of watts of power needed to achieve that air movement will be less with a more efficient speaker, but not why, if it's making the same SPL in-room, there would be less excitation of the cabinet. Anybody enlighten me on this one?