Why did you reduce the pot value of 5k to 1500 ohms as a resistor? Remember, the NFB is a voltage divider from the signal voltage at the speaker.
27k over 5k is a roughly 6/1 divider. A 27k over 1.5k is more like a 20/1 divider. SO that change alone makes a large difference. BY adding the pot, you can then reduce the NFB even further. meaning it will have a much smaller effect.