I also own the Z560s, and the volume is just right. My sound card is an SB Live! 5.1 POS.
There is some variance in the volume between different drivers, i.e. Creative's drivers for W98, WXP's default drivers, and Linux's drivers.
If the speakers are pumping out an obscene amount of bass or raw power, then I think the audio drivers are partly to blame.
My volume is set just below half, and the bass is at half.