Audio Amplifier Design (CE Amplifier - Class AB Power Amplifier)
Q3's Emitter and Collector are the wrong way around. The Emitter should go to R9 and the Collector to V3.

The power output stage (Q2/Q3) has a voltage gain of slightly less than 1. To get full output voltage swing it must be driven close to both positive and negative supply rails. The input stage (Q1) is powered from +12V only, so it won't be able to produce the +-12V signal required to get full output.

The 8 Ohm load draws a peak current of over 1A at full output voltage, too much for the 2N3904/6. Also resistors R6 and R7 won't supply enough current to drive them at that current. Try a higher load resistance (eg. 50 Ohms) to get more output voltage without distortion.