I've heard of those - iirc they need a bespoke amplifier to power them. The K240's, despite being 600 ohm headphones, work and sound great straight out of an old iPod - although they're rather quiet without using a dedicated headphone amp.
I seem to remember about 1980(?) the Stax were around £1k. For me back then the K240 'sextet' were, IIRC, about £30, which was quite a lot of money, but today the K240 MkII are £66. That alone tells a story. You can almost forget that you're wearing the K240s as they're so light. The Stax needed a headrest to avoid getting a stiff neck.
Yes, the Stax have a bespoke amplifier+power-supply as the electrostatic drivers need a high voltage DC supply. I presumed the amplifier came with them, but I see they're listed separately ... today's "flagship bundle" is $11k.

My K702s have been fine ... no rattles or squeaks. Yes, bigger, heavier and a little less cool. I do think they're also not as good as the old 'sextet' 'phones were. The old 'sextet' models looked cheap for the money and I wonder if that hit sales.