I couldn't resist so I took a chance and changed the FET's; and guess what, everything is in working order again!
They indeed where the problem. Changed all of them, took 20 minutes disassembling and everything.
Maybe it was a c**p shot, but I had a feeling they where bad, since the one on the power distribution board was bad in the beginning, and by looking at the service manual, it seemed like they very much are the centerpiece of the preamp.
So for future problems for other cp70/80 owners with bad pre-amps; check voltages and check/change the FET's, and hope you find this post
Thank you so much guys for second opinions and advising me!
All the best greetings from Peter
