You hit upon it - the wavelength of frequencies below 40-50hz is far beyond the size of the cab, so it's largely going to just sound one note-ish below that anyways. It'll eventually rattle the entire cab apart as well - because that sound pressure has no place to go in such a small space and then starts to vibrate everything that's at or near its resonant frequency. In a 4Runner, way in the rear, it would probably make a difference, because there's just enough space inside the shell to get some bass extension that's not muddy.
At home, with a large enough room, it's great, of course. But cars are a small and usually pretty tightly sealed environment, compared to a typical living room. Rattle the windows, feel the bass in your chest sort of thing - but not really doing you or your hearing any good. Those 7 inchers are actually kind of overkill. At under $50 a speaker, it's a killer deal as well.
No - they really do sound quite nice. Spend money on fancy Alpines or Infinity or whatever if you want - the Kenwoods are 95% as good for a fraction of the price. Better than JBL's car audio and Bose, btw. (note - NOT true for home audio - heh)
Oh - movie theater subs are set at roughly 35-40hz in order to not cause long term damage to the buildings. They make up for it in volume and air moved, of course. But it's hardly musical.
