DC Offset alignment.
1. Warm up 5 minutes min.
2. Set volume at minimum.
3. Connect voltmeter across left speaker, 100-300mV DC scale, speaker positive to meter red.
4. Adjust VR5 (left) for meter zero.
5. Repeat for right channel (VR6)
VR5, 6 trimpots are 20k on my schematic.