Due to the fact that music, or any recording short of straight pink noise or a constant tone, will have dynamic peaks. The volume you can go to on an amplifier will be limited by these peaks and the amount of dynamic head room the amplifier has to handle them. The Denon receiver we have at our lab, on normal source material, goes into clipping above +04 on the volume control. This would seem pretty consistent with what you are finding also. I think if you want more output you will need a larger amplifier.
