Hi,
I'm fighting with the same sh*tty circuit, only from a JAMO SUB650... Clearly some kind of chinese crap. Anyhow, there are two amps working in bridged mode and are straightforward AB class with a pair of mosfets on the output.
The PSU its a little bit more tricky... There is the input converter which is an self oscillating half bridge circuit. It works immediately when you switch on the board and spits out some 80Vdc at the main smoothing capacitor! And here is where the fun begins, there is a kind of switching circuit - I traced down part of the circuit - which is connected somewhat unusually. What happens is, that the amplifiers get the full supply voltage instead of getting a variable supply according to the output power requested...
I replaced the small IC on the daughter board - UC3842A but still nothing. There are two fat resistors 150ohm 4Watt each, used to achieve a virtual ground - you see, the amps are powered with single ended voltage - that start to smoke since they are not able to dissipate the power delivered from the main PS. They are connected in series and the mid point is the virtual ground for the rest of the low power amp circuit. Being 300ohm connected straight across 80Vdc they should dissipate some 22Watts(remember the two are 4Watt/each). So this is the clue to me, that the switching circuit(yess indeed there is a large inductor...), somehow LOWERS the supply to the output amps, when full power is not needed...
Of course I contacted Jamo and of course they very politely told me to bug-off...
Jamo used to be a serious manufacturer but not anymore it seems.