This Raal Requisite SR1a headphone was pretty interesting. The driver is user replaceable (about $390 I think):
View attachment 28096
View attachment 28097
View attachment 28098
I listened to them for a while. They don't sound like headphones or speakers. Actually they sound like little speakers hanging around your ear, pointing back. You can fold them to get better bass. I thought the sound was open (well duh!) and pretty low in distortion. Sounded really good for classical music.
But again, they are a unique experience of their own. Worth trying them even though they are pretty expensive ($3,500?).
They have 0.2 ohm impedance so you need a power amp and their adapter to drive them.