Chinese amp with Chinese transformers. So:

a) Obtain a dental-tool, one of those devices with a sharp, pointy hook on either end. With this tool, check _EVERY_ connection inside the amp. Look for cold-solders, no-solders, cracked solders and so forth.

b) https://www.stereo.net.au/forums/top...d-ms-10d-mkii/ With that in mind, it is unlikely that the tubes within that beast have much to do with your issues, so now go to the boards and look for any faulty connections, over heating components (freeze-spray is your friend), raised traces and so forth.

c) Once done with the physical examination, do obtain a metered variable auto-transformer and determine two things:
i) At what voltage does B+ kick in? Given a 5U4, should be about 80 V+/-
ii) And there should be a definite hard rise in current at that onset..
NOTE: You should be drawing within 20% of +/- 50 watts or so, quiescent. If you are drawing substantially less than that, the tubes are eyewash, not functional.

Good luck with it - you will likely need it.

Peter Wieck
Melrose Park, PA