They seem reasonably priced, if they give you that quality.
I've not seen speakers mounted in the head rests before; is that much work to do? How does that go with the proximity to your ears?
All V-Spec MX-5's have speakers in the head rests:

You can see them here.
The headrest speakers are on the back stereo outputs of the car audio system (which usually comes with front stereo and a back stereo outputs). Then you balance the the audio slightly towards the front speakers. It sounds great, especially when the roof is down and you need the sound reinforcement

The purpose of the headrest speakers is to transmit the high frequency audio that is lost when travelling at speed with the roof down, and the smaller CS-V425s in the head rests also have a nice solid bass on them which gives the seat a bit of bass punch too
