The overall design should work at lower supply voltages but many of the component values will need to change. For example, the bias resistors will need to be changed to get the appropriate bias currents.
This is a push-pull design. Lowering the supply rail voltage is how you get lower power output. Consider the case when the upper output FETs are fully on, the output will be driven to +70V. Typical speakers are nominally 8 Ohm so the output current will be 70V/8R=8.75A. P=IV so P=8.75A*70V = 612W. The 400W rating presumes that you aren't driving it to the limits to keep the distortion low.