The first thing I would do is replace the rectifier tube (assuming it's tube rectified and that the tube is of Chinese manufacture).
A quick Google showed an inexpensive Oldchen single ended EL34 based amp with a 5Z3P/rectifier (2) 6N9P small signal tubes and (2) EL34's (one per channel).
Is this the amp?
A failing rectifier tube can wipe out the EL34's plus other passive parts of the amp which is why I would start by swapping in a known to be reliable tube.
Reasonably priced one's (for 5Z3P) would be a Russian 5C3S (all I've seen were manufactured by Svetlana) or a GE 5U4G (both run around $30, while other brands start @ $100+).
That done I would just listen to it for 3+ months before changing anything else.
The 6N9P signal tubes are equivalent to 6SL7 (not familiar with current prices and/or what people like for this tube).
I'm not Anti-Chinese tube on a whole as I've heard some of their EL34's, 2A3's, 300B's and 12ax7's that sound quite good, but their rectifier tubes seem to have a bad rep for reliability.
Before you start tinkering inside learn how to safely discharge the power supply capacitors.
DeKay