In case you need to do this for another amp the quick way is to just determine the voltage limit (it’s very unlikely an amp will be current limited at 150 Ohms).
Just take the highest impedence rating 600 Ohms 375mW and use V^2/R = P.
V = SQRT(P*R) so SQRT(0.375 * 600) in this case 15V RMS.
You can then just type the numbers into a calculator like this one
And you’ll get 109.76dB as the maximum volume as you calculated.