The reason why electrostats «generally» (= averagely!) sound better is: they are niche products for demanding people. They can't be plugged in every headphone jack, need special devices to operate and thus never get a wide spread, so they never reach high production numbers, meaning they are expensive anyway. So a cheaply built electrostatic would make no sense. Technically there's no reason why they have to be more expensive than dynamic headphones. Apart from the voltage supply/transformer unit their design is even simplier.
If this wasn't the case there would be a lot of mediocre sounding electrostatics around, maybe in the form of electrets.
Nobody can deny that (good) electrostatics have some qualitites dynamic types barely can equal. In terms of detail, transparency and freedom of obvious resonances they're simply the best. But when it comes to the bass, there's no competition. Well not every dynamic headphone has better bass than the best electrostats, but it's a valid generaliation anyway. With «better bass» I don't necessarily mean «slam» and visceral impact, but the «grip» and control normally only dynamics provide.
The best example therefore is the Etymotic ER-4. Its bass has no visceral impact at all, and many people think it sounds thin as paper. I for one like this bass a lot, for me it's the most controlled and accurate bass I ever heard. Well, it comes from the tiny membrane of a canalphone which is directly coupled with the eardrum by a tiny air volume, so you can say with low frequencies the eardrum is directly driven by the transducer's voice coil.
O.k. – this is a case where we have indeed very low moving masses even similar to electrostatics. But I don't think this is the clou, but in fact the direct coupling membrane/eardrum. However, the conclusion from this example is: the Ety bass sounds very different from electrostatic bass! There's a lot more control and speed. Electrostatic bass is slow (sorry for the generalization, of course there are differences among them...). It is, in the best cases, very clean, transparent, unobstrusive and non-fatiging. But it's slightly uncontrolled, at least compared to the best dynamic headphones. The best exponents of them (which are usually open designs and clearly not some with typically boomy bass) come much closer to the Ety bass than the best electrostatics. Of course this statement is the result of my personal, limited experience.
The superordinate reason for this example is to show that the preference for dynamic bass isn't synonym to preference for distorted or boomy bass, not even impactful bass.
But it's not just the bass which is concerned by this lack of control and «grip», but also the midrange and even the highs, to a certain degree. This is the reason for the more colorful, more «musical» reproduction of dynamic headphones. It's as if the music is more substantial, more real, whereas the electrostatic sound is more of an etherial, intellectual or analytical kind (roughly spoken). It's audible that there's not a rigid, massive membrane driving the air, but a soft and thin foil.
The resolution of electrostatics with mid and especially high frequencies is simply unbeatable as far as I can imagine. Every tiny detail is reproduced with ultimate delicacy. Beside the openness of most electrostatic enclosure designs with a minimum of hollow colorations and thus smearing of the original spatial depth information this may also be the main reason for very wide soundstage most electrostatics offer. The downside of the lightweight and sound-leaking diaphragm: every change of acoustic impedance immediately affects it seriously. I.e. if you hold one finger over the driver outside, there's a huge sonic impact caused by the corresponding reflections. Even the typical vertical plastic trusses on the Stax enclosures have a considerable impact on the sound (the reason why I have used much thinner grid for my own constructions). Knowing that on both sides of the diaphragm there are stator grids to provide the driving force in the form of high tension which BTW have less than 50% air permeability, thus a high acoustic resistance and reflection potential, you can easily imagine how much this influences the sound. The ideal electrostatic transducer has no stator grids at all...
I put these theoretical speculations in front of the thing I'd actually like to explain: to my ears, the treble of electrostatics, despite its ultimate clarity and focus, has some sort of glaring accentuation of brilliance, even if the highs themselves are not overly accentuated. I guess the reason for this are the stator grids. I even state that to a certain degree this effect is also responsible for the ultimately high detail and resolution one perceives with them. But this is only speculation so far.
It may seem as if I hate electrostatics. Quite the opposite is true: I like them a lot, and if there wasn't a certain Mike, I would defend them against every unfair criticism. Once you have accostumed to the opulence of detail, you realize: in a wonderful way it's simply not fatiguing, you can listen as long as with barely any dynamic headphone. Well, Im not speaking of the Lambda Pros I had years ago... those had a sharp and peaky treble which made my teeth hurt. And to come to an end: the liquid midrange is so wonderful you feel like swimming in the music... Pity I have decided to like dynamic headphones better.
JaZZ