I am not too sure about how old they are, I looked through a bunch of old Philips catalogs and didn't see them but I doubt because of the model number they would be any earlier than late 80's. SO late 80's or up. I don't have a box to look at.
The reason I put a different cable on mine ( which were the exact same 6.3mm TRS cable as yours were before ) is that running them directly off the speaker terminals would be better, but since Stax makes electrets and I also have a few adapter boxes the Stax headphones hook up to, I replaced with a Stax Electret cable. ( note: not full bias cable )
These Stax Adapter boxes ALSO run off speaker terminals but have the big transformers inside to step things up beyond that so these sound much better now. I also did a bunch of mods to bring down the treble peaks.
I also removed the tiny transformer in the earcups which realllly limits these and causes distortion if pushed too hard. They crap out. Another person found the same thing with some Audio Technica electrets ( specifically AT-707 TRS jacked rarities ) that had the same tiny transformers in the earcups. They also caused distortion.
There is a "Silistor C" TS-4601 thermistor ( thermally variable resistor ) on the Stax cable end right inside that black bulge there. That is stock on the stax cable.
Filled the rest of the cup space with some special acoustic filler to add some body to the music.
After all this and plugging them into a Stax adapter they sound night and day different than stock.
If anyone decides they want to sell their stock pair let me know :) This is a bit of an expense to do this mod unless you have some scrap spare parts lying around like I did, and of course the adapters.
Edited by nick n - 8/26/12 at 4:54pm