Why the circuit behaves in this way?
It's a poor design.  


LM741 current limiting  is causing clipping also has 2V saturation from Vcc,Vee  . Get Darlingtons
If these 9V were batteries the 1 uohm would be >=10 ohms
The caps are not needed , except ADD Zoebel RC Snubber.
C8 is not required. 
2SB1258 2SD2642 may be good options for Darlingtons or DIY equiv
below I showed quasi PNP using power NPN, signal PNP
power NPN 10A for example


For more output, use Rail to Rail  CMOS OP Amp with Darlingtons


6W pk 3W RMS 8 Ohms
REV C
 
 200R represents current limit on Op Amp

Rev D Gain =5 (fixed err.)



If still using poor LM741 OA , you can reduce Darlington saturation voltage   using Quasi-power pairs with Power PNP,NPN to make the opposite  NPN-PNP Simulation HERE