Cheers Dusty Chalk- I'm exploring the insides of my old PX 200s just now.
I have the book 'The science of sound' by TD Rossing, FR Moore and PA Wheeler which is good but fairly basic, 'Loudspeaker and headphone handbook' is much more detailed but to my enormous frustration after finding it was on google books (too expensive to buy) they had cut out the headphone pages!!
Thanks for the advice.