Had another round of cable management in my cupboard, main goal was to further reduce the interference.:

  1. Moved power supplies of all other units in the cupboard to a different socket.
  2. Created space between the amp and other units in the cupboard to reduce
  3. Moved all irrelevant wires away from the amp
  4. Finally, I arranged the slack of the long wires to create a twisted couple, then rolled the into a smaller loops

The amp is whisper quiet at max volume now, virtually no hiss, no hum. Wow, never had such a clean sound from the amp. Super happy.

What's strange about my amp is that the top cover is painted both inside and outside, it doesn't have any contact with the grounded base part of the amp. I am thinking of shaving the paint under the screws that connect it to the base, this should effectively ground it. Would it be a good idea or am I missing something big time?