Hi Birk,
In the mean time I did some more work on this amplifier... hum still present, nevertheless I am now sure it comes from the first stages... maybe bad grounding but I will take a break for the moment.. here are some pics maybe helps... The wiring is not the best one even if such small place available.... Maybe I will redone this part. I made as well some additional measurements.. With 1Khz / 1,2V at input I get on a 8 Ohm dummy load (100W) a little over 59 Vpp before clipping... anyway not the power is my target but the quality of the sound reproduction... I consider this setup rather conservative and I think will be in the benefit of a very long life for the tubes
Best regards and happy building....