An interesting thread from 2007 was bumped on this very subject.
Short answer seems to be: Stax in 1960 with the SRA-4S, as electrostats require a special amp:
http://www.stax.co.jp/OLD/Photo/SRA4S-1.jpg
but for dynamic phones, excluding pro gear with headphone outs, probably the Sure Solo-phone.
http://mysite.verizon.net/hirsch9000/shure1.jpg
After that, Grado designed the HP-1 for the HP-1000s just before HeadRoom started up business in the early '90s and the Melos SHA1 predates HeadRoom as well. HeadRoom created the first portable, of which the Desktop Portable is a descendant, and also the first balanced amp, the BlockHead, consisting of two mono amps in one box.
Short answer seems to be: Stax in 1960 with the SRA-4S, as electrostats require a special amp:
http://www.stax.co.jp/OLD/Photo/SRA4S-1.jpg
but for dynamic phones, excluding pro gear with headphone outs, probably the Sure Solo-phone.
http://mysite.verizon.net/hirsch9000/shure1.jpg
After that, Grado designed the HP-1 for the HP-1000s just before HeadRoom started up business in the early '90s and the Melos SHA1 predates HeadRoom as well. HeadRoom created the first portable, of which the Desktop Portable is a descendant, and also the first balanced amp, the BlockHead, consisting of two mono amps in one box.