Power consumption problem in high-voltage power audio amplifier
Class B has a maximum theoretical efficiency of 78.5% at maximum undistorted sine wave output. However at lower power output the efficiency drops because there is more voltage across the transistors and less across the load. At very low power the quiescent power consumption also becomes significant, so a high power amp will have lower efficiency than a low power amp at low output.  

Here's an example of efficiency and power dissipation vs power output for a 12W audio power amp IC (TDA7240):-



We see that power dissipation actually increases at lower output power, and is the same at 1.5W output as it is at 12W. 

An amp that delivered 500W into an 8 ohm load at 75% efficiency would dissipate 167 Watts. It might also dissipate 167 Watts at 60W. Your 24W for 10W output sounds quite reasonable.