In case anyone ever needs to replace the headrest speakers in their vert, I have found that 3.5 inch Infiniti Kappas are superb. There is a bit of work to do to make a good fit, but it's quite doable. You won't, however notice a huge difference in the quality of sound..they are tiny speakers, after all, and the stockers are not really bad at all. I suspect that the power handling is now much improved with the kappas.
If anyone needs details on what I did I'd be happy to go into detail, but it seems that replacing the headrest speakers is quite rare in the RX-7 world.

 

Don't have headrest speakers in my rex, but I put a set of Clearwater HR speakers in my Miata, and was pretty impressed... Think I got them thru Moss Motors. Don't know if they'll fit a 7, but may be worth looking into as well.