actually my friends are also in trouble with this circuit design
[ Ri:50kohm Ro: <0.25ohm output power: 20w @Ry=4ohm gain: 40 dB THD < %0.5 Bandwidth: 20Hz-20KHz ]
can you build one more extremely simple circuit? i feel cheeky but it sounds it is easy for you.

actually i memorized the steps, have a large gained amplifier first, then feedback it, then add ab class power amp. feed it with enogh Vdc, arrange the capacitor values. thats all. but i am sinking in details and i cant do it somehow.

im going to send it output wavefrom. it takes time because i am connecting internet from another computer.

you mean this?

sorry, here is output power 20w into 4 ohm,

 

Look on the imagine which I add

 

now it says 7.32 percent, (at the end of the output file)

 

The output waveform shows pretty bad distortion at an output of only 15W into 4 ohms.

You did not show the results of the Fourier Analysis to calculate the amount of distortion.

If R9 has bootstrapping then the distortion will be much less and the output power will be higher. R14 and R15 should have a higher value, try 2.2k ohms.