Why aren't there any active headphone designs? That is, a headphone with a matching equalizer, and probably an amp.
I am not an engineer, but it seems to me that active design would be even more useful in headphones than in speakers. Headphones are like single-driver speakers, there is no crossover. The only way to change the response is to make mechanical tweaks, or electromechanical tweaks in the motor. That must be a difficult and arcane process; every time you try to fix one problem, you probably introduce others. In EQ circuits, on the other hand, it's straightforward to alter the response in a particular way. Also, headphones aren't even designed for flat response. You're trying to create a certain curve with peaks and dips through tweaking the driver.
It seems like it would be much easier to design active phones. Got a great phone that's light on bass? Boost the bass. Need a peak at a particular frequency in order to get to your target curve? EQ a peak. And so on. Why don't they do that? Does anybody do this, or do any headphones have passive EQ circuits built in?