How does it work?

Mainly, I receive headphones to work on them. I do not supply them.
The primary reason for this is that people send me all sorts of ‘unicorn’ vintage Sennheisers that they want me to work on.
Are there sonic differences between vintage and modern? Yes. So this is how it usually works.
The secondary reason is that people like how a certain stock pair they have sounds, and want them upgraded by me. Because, each pair sounds different, no matter how good Senn matching is. (And I can tell you their matching is really top notch.)
Sometimes I am able to source pairs locally, and some people have opted for this option.
This is not a set thing, so this is conducted in a case by case basis.
A long time ago, I used to supply pairs myself because I wanted to be able to offer a single consistent sonic profiles. But this has now proven impossible because of some changes to how Senn operates (especially how parts are much harder to source now).
Also please read the FAQ and other sections of the site to fully calibrate your expectations.
Don’t be disappointed if you expected a HD650 sound out of a HD600 or something like that, or vice versa. Obviously if you expected a HD650 sound you should be sending me a HD650, not a HD600.
My job is to help these headphones be at their best, in different styles and presentations. I can influence them to become a certain way, but I don’t alter their character and so their final sound depends a lot on how they were stock.
As a friend put it to me: “JAR- We turn horses into unicorns, but not into different animals!”