The brightness of the Watt comes from its tweeter's dome break-up that is too close to the audioband. It is not brightness, it is simply distortion. Some, may find it exciting.
Almost all extremely rigid light weight drivers seem to run into resonance problems (high Q) - it is physics after all - and even when the problems are supposed to be "out of band" they often appear to affect "in band" response, especially at modestly elevated SPL levels...just my two cents....a cone that does not internally damp itself is akin to a "bell". The "bell" acts like an instrument in that the way it affects the sound can be quite unrelated to the music and therefore a more noticeable form of distortion. Soft damped cones have other distortion breakup mechanisms and they are much heavier material and require more expensive drive motors but they they have a big advantage in that they tend to damp unwanted distortion that is unrelated to the music.
I can't think of an easy way to explain it except that TIM distortion is much worse than Harmonic distortion and "ringing" drivers tend to produce TIM-like distortion.....much more audible....two cents, as usual.