Why no amount of watts will compensate for direct-radiation :
Start with the obvious : When compared to either multiple direct-radiating drivers or a single horn-loaded driver, to achieve the same SPL, a single direct-radiating driver must work harder; its membrane must move more, its "stroke" must be longer.
If the membrane of the single direct-radiating driver is to keep time with the music, it follows that the speed with which it moves through this extended stroke must be greater than that of the relatively short-stroke multiple direct-radiator or horn-loaded driver.
Due to this greater distance, and need to keep time with the music, the moving mass must be driven at a greater speed. As stroke increases, this speed goes up fast; with speed comes inertia. The materials are suddenly being physically solicited beyond their limits, so they are reinforced and mass again rises.
In the end something must go, and the natural result is that the smaller movements (smaller currents through the coil) just get overrun. Even with a very powerful motor, a membrane driven under such conditions simply does not have time to reproduce the smaller movements.
Ways around this issue : Get SPL via increased membrane surface area (multiple direct radiators), or horns.
I suspect that some of what we call tone comes from what is in fact a natural resonance set up by the light-weight, stiff membrane material and its lightness as "seen" by the magnet and coil. Micro-managing a reinforced membrane with increased force is not the same thing.
How to short-circuit evolution: Enshrine mediocrity.