I have made some more Tests directly at the IC's of the Pre-Amp. At all of them the +15 and -15V and also the 48V are available. Then i took a amplified PC-Speeker an plugged it to the Thru-Plug. That is working. Music was comming out of the PC-Speaker. So the Pre-Amp is ok.
But there is no noise from the b205d-Speaker. The PWR-Amp is not working.