Got it sorted my crack is working now
I did some more measurements with a different DMM but this has not shown any difference. Looking at the wireing also did not reveal any problems.
Then i tried to check the tube pinout to be able to validate that the tubes are working.
The information i found was that the tube should be feeded with 6.3V at pin 8, so i took a closer look and i saw that B8L was not soldered o.O
Rechecking the manual revealed that there are no wires missing on B8L but that there is no clear instruction to solder B7L and B8L, maybe this should be added to the manual?
Thanks for your help!
ps. enyoing my Crack now with a Sennheiser HD600 attached!