[KaiSc]

It is believed that the earlier the serial number the better the sound since there were some changes made in the production process over the years to make the headphones cheaper to produce. And that is also my experience. Also on Ebay early serial numbers are usually more expensive.

According to Heinz Renner (ex AKG, now LB-ACOUSTICS.AT - Mysphere) the transition was at about S.No. 5000.

The service manual of the later version dates from 11/1990.

Changes where done:
• Higher resonance frequency - “Bass-Light Tuning”.
• Circuit board layout changes, but same circuit / schematics.
• Voice coil contact wires now run at the top of the diaphragm dome, this is visible through the wire mesh.
• Mechanical changes in the frame and swivel.
• The service manual mentions variations in the cables’s wire colors that are not related to manufacture generations.

Bass Light Tuning / stiffer suspension likely was introduced to achieve higher SPL without the diaphragm hitting the limit.
At that time people tended to really listen very loud, far beyond healthy.

The other changes might have helped to simplify the manufacturing process.

 

[KaiSc]

And you shouldn't. I sold my pair with a very low serial number (1xxx) made in 1989 in like new condition, and I feel now that it was a big mistake. The price was great, but that pair sounded better than four others I compared it with. I do now have a pair of the Mysphere and it is terrific headphones, but the K1000 is also unique. I miss that sound.

Both have their place in my collection!

The Mysphere largely benefits from EQ, look here for my 2 personal settings:
https://www.head-fi.org/threads/mys...und-design-build-quality.819658/post-17398912