7.) Power supply caps replacement with min. 500V ones - Reliability, Safety
8.) Replace capacitors with higher grade electrolytic or polypropylene capacitors (Nichicon, Mundorf). - Reliability
9.) Replace all resistors with Vishay PRO metal film resistors - Reliability
10.) KT88 heaters wires rerouting and removing grounding jumper from heaters - Reliability, Safety
11.) Global Negative Feedback - GLNFB - Ongoing... I have tested with 24K resistor and 220 pF MICA cap but it my case that does not work correct. The ringing on sqaure signal is gone but the output power dropped in UL Mode from 38W to 22W.
After all job done I'm pleased to confirm everything that Stephe said or demonstrated in Willsenton R8 series.
Thank You Stephe for posting all these valuebale videos and documentation!!!