Yes, I calculated since it runs on 32V a 1Vpp signal would be
amplified to 30Vpp at highest volume thus getting the maximum
undistorted swing at the output. I wonder why the designers didn't
make this simple calculation.

Actually that would be for a symmetric supply, but in this case it has
single supply so the maximum swing is half (+/- 16V). I used a pair of
1K8 resistors placed in series with capacitors at feedback pins 2 and
7 to set amplifier gain to 15. Volume regulation is now much better.