Just looked up Dharma D1000 on the headphone-guru audio review & they detailed it. For Dharma - the electret element kicks in at 12,000 Hz, so it hardly covers much bandwidth upward, providing "air", spacialty, overtones, highest bells, triangles, very-hi-cymbals, etc. They said other reviews mistakenly printed the crossover at lower Hz, but Mr Wei Chang (EnigmAcoustics) states 12 KHz. Because it does not need to cover the power demands of the full bandwidth downward, the embedded precharge is sufficient to cover the far upper frequencies alone thru self-biasing (what "power" is coming in thru the cord, from source). These hi frequencies have little demand for power. Power not the best word. This is because the paper cone dynamic driver covers almost all of the entire frequency response - covers 20 to 20,000 Hz itself alone. The supertweeter effect of the electret element covers 12 to 40,000 Hz, per Mr Wei. So, Dharma does not need transformers, confirmed by 6Moons Audio also. This design is patented by EnigmAcoustics. Nice design-model.
The electrastatz crossover point is somewhere within 1000 Hz to 2000 Hz area, in the midrange region where much music lies. Some audiophiles are very picky where they prefer crossover points (in loudspeakers) to be, especially classical music listeners. The transition area can be incongruous between drivers, especially for strings and piano, and it is a touchy subject. Strings & piano, along with other instruments playing phrases starting at one side of the crossover & ending on the other side (think of a simple glissando) can sound disjointed thru the musical passage. It's where an instrument extends through playing across 1 driver into and thru the 2nd driver- sometimes the poor driver transition is very audible if designed badly. Probably why Dharma puts their's at a supertweeter frequency crossover.
So, this particular permanent charge electret, patented only in Dharma use, needs no transformer. However, ALL electrets are permanently precharged, and all need transformers or electrostatic amps to drive them, but not Dharma.
Probably confused the explanation & see your question. Because (exception-not Dharma now) all electrets need some extra "juice", they need a transformer somewhere, or an electrostatic amp providing upconversion in higher voltage. Almost all electrets used an external transformer box (ex: like Stax SRD-4 box, etc) to provide that extra juice. Audio Technica & maybe 1 or 2 other vintage models put small transformers in each earcup so we wouldn't need that external box - their cord terminated in a standard plug. A-T and very many other vintage brands (Philips, Toshiba, Aurex, etc), also chose another way, which was neither transformers inside cups or and external box. Those many types instead put the transformer in the six foot cable-cord as a big bulky end plug which housed the transfomer along with the plug. These plugs are about 3 or 4 inches long and 2 inches diameter - it terminates with a 1/4" male phono plug to go into headphone jack of source. Often referred to as a "bulge-plug", it allowed convenience of no external box, and also portability. So, they all needed some type of transformer or electrostatic amp to drive them. Example - Stax Sr-30 Pro electret runs off a SRD-4 external transformer "energizer" box wired to rear loudspeaker-amp speaker-out-taps. The SR-30 Pro electret can also run off any stax pro-bias (580 v) or normal bias (230v) electrostatic amp, such as a Stax SRM-1/Mk II Pro model electrostatic amp.
All electrets posess a permanent embedded charge at manufacturing. True-electrostatics (ex : Stax SR-Lambda Nova Signature, etc) do not. Those true-stats require the charge provided externally from a matched stat-amp, or an external "energizer" box connected to loudspeaker amp (Woo Audio Wee, Audiovalve Verto, illusion esc-1001, etc). We also refer to these "energizer" boxes as a "transformer" - because that's what's mostly inside of the box. My Woo Wee (have illusion also, just bought both used) contains heavy, massive transformers - 3 - I believe, last time seeing nudies of the inside with cover-lid off recalling the insides. It weighs a ton - very heavy for its size.
So I understand your deciphering of this and any confusion. Dharma is much different with their patented design. Summary - electrets all are precharged permanently & need either transformer(s)-either as energizer box or bulge-plug, or small ones in-earcup, or stat-amp to drive. True-electrostatics have no permanent charge & need either matched stat-amp or energizer "transformer" box to drive. Sorry for confusion.
Dharma is different & the tiny needs of their electret kicking in at a hi 12,000 Hz needs no extra "juice" because they used a very strong permanent charge (patented) only needed for this very high-bandwidth area powering. The regular "juice" coming thru the 6 foot cable along with the music signal is enough to drive that electret well. Noted is on average a young human hears generally to about 20,000 Hz. At age 35 or 40 roughly, that avg human now has hearing capped at 14,000 Hz on average from physiologic aging - happens to almost every one of us. So Dharma electret supertweeter range of 12KHz to 40KHz is mostly for "airiness" & spacialty. At 12 KHz up, the instruments are limited -like triangles, very hi bells and tiny-hi cymbals, overtones, really high notes on violins & reeds like oboe-clarinet- plus piccolo & flute, and some other things like really hi synth notes. (think really hi, like ultra-squeaky) !
Maybe this all clears up what drives these elements.