Good technical idea for small enclosure box, but low manufacture quality control and low grade components management inside . Separation power supply and audio output section will help a lot , moving cooling fan to center will help also , not using SMD capacitors will do triple help . This unit repair is all about soldering joints RE FLOW for output amp/power supply section board .
3 layers SMD PCB board with lots of heat emitting elements around on small perimeter board is not good combination . Cooling fan helps but not enough .
After output board reflow unit start working ok . Reflow must be done on one and other side of PCB board.