how to increase output power of multistage audio amplifer?
You want to increased output power without increased power supply voltage? Are you willing to have the lowered load resistance;  i.e. multiple parallel connected speakers at output?

If not there is no point of any sort in discussing modifying the circuit.

If you are willing to have the lowered load resistance, your circuit need to be modified to enable it handling high currents at it's output  stage. To do this, add base resistance of 100ohms to each output transistors and repeat its counts to 4 pairs.

That will give you 0.5ohm capable output stage. That's 100 watts with 12 volt  power supply.