Class AB power amp output
Your opamp U1 does nothing and can be replaced by a piece of wire.
If the circuit is perfect and the battery is brand new then the output voltage swing will be 9V peak-to-peak. Then the undistorted power in an 8 ohm headphone will be 1.27W and into a 32 ohm headphone will be 0.32W which will be extremely loud.
But the opamp cannot produce an output voltage swing without a voltage loss.
The opamp U2 does not turn on the output transistors, instead it turns them off.
R4 and R5 turn on the output transistors.
The OP07 opamp has a typical maximum output swing of only 2.4V peak when it has a 9V supply. Then its peak output voltages are 2.1V and 6.9V.
The output to the headphone will try to be 2.4V peak which is an output current of 300mA into an 8 ohms headphone. With a collector current of 300mA a BD135 or BD136 transistor has a minimum hFE of 30 so the base current must be 10mA.
Each diode adds 0.7V to the peak voltages at the bases of the transistors to 1.4V and 7.6V then resistors R4 and R5 have a current of only 1.4V/2.7k= 5.2mA which is not enough if the transistors have minimum hFE.
Assuming that the opamp and transistors have typical spec's then the 2.4V peak in the 8 ohm headphone produces a power of 0.36W. Turning up the volume will result in severe clipping distortion. If the opamp and transistors have minimum spec's then the maximum undistorted output power will be much less.
Using transistors with higher hFE will not increase the maximum undistorted output power.