Probably been posted, but here's a bunch of KOSS Pro/4AAA drivers. There are 3 different series of drivers, and are in EP, MP, and LP production models respectively.
Series 1:
Original KOSS Pro driver from the mid 70's. Very intriguing design with a large voice coil and a fairly thick diaphragm that isn't glued down, but instead held in place by a metal ring. The plastic housing follows the shape of the diaphragm, leaving a small gap of air between the two parts. Has a very large, heavy magnet. Great soundstage, incredible bass, lush mids. Somewhat recessed treble. Very detailed. A very powerful sound with no peaks or resonances. Original paper damping deteriorated and was replaced with fleece.
Series 2:
Redesigned version of the series 1, in production somewhere between 1978 and 1979. Rarest of the three. Has three bass ports instead of four, and foam damping instead of paper. Diaphragm has the same shape but is very slightly thinner. Thinner plastic housing, but a larger magnet. Sound is the best of the three, same sound signature as the original 4AAA but with <MUCH> lower distortion and considerably better treble extension. Has better transient response due to the better magnet and lighter diaphragm, and soundstage is larger as well. Literally a straight up improvement.
Series 3:
Newer driver from the 80's, completely redesigned. Much plainer in appearance. Smaller voice coil and thinner diaphragm. Smaller, lighter magnet. Bigger soundstage but worse everything else. Peaks everywhere. Has the horrible tin can veiling so common in headphones from the 80's. No detail. This driver was used in all "Plus" models of KOSS headphones, and was in production until some time during the 90's. This is the driver that killed KOSS's reputation as a headphone manufacturer. Funny how they put it in the best headphone they ever made until the ESP-950.
