post net list
If you would post the net list, I will give it a try in PSpice. You should specify the output transistor model numbers and all other transistors that are not specified. I suspect that the circuit was experimentally cut and tried for a specific speaker with its specific impedance vs frequency characteristics and that is why you may be having trouble. Please replace Rpot1 with a fixed value resistor which we can tweak in the net list. Post the net list in .txt format and I will give it a try.
When the circuit does not work, what are the symptoms, (oscillating at frequecy {specify the frequency} or going to the supply limit {whiich one}?