a) The silver parts are made of metal. The black ones are made of plastic. I suspect the meshed plate of the earcups to be made of magnesium alloy. Oh, and the T50p were in metal too (in fact there is more metal in the T50p as they have relatively less plastic parts).
b) Jude said they change quite a bit depending on position. Though with my experience with the T50p it has much less to do with actual positionning than with "seal". You have to think about those as quasi IEMs : a good seal is needed to get the sound right. Otherwise they'll sound shrill. When you move them around, there will be a sweet spot when they suddenly come to life, and that's because of the seal. BUT I've increased the "hit rate" by simply bending dramaticlly the T50p headband to increase pressure. Now it never bothers me and I always get a good seal. I read somewhere that the DT 1350 might come with a higher pressure headband out of the factory, but you can bend it further as you wish.