Even outputting 10-20 watts per channel, you need a whole lot bigger amplifier, like 100-200 watts if you want good clean undistorted sound on the lowest Bass peaks. If your amp cannot supply the needed watts on the peak bass notes, it will distort, which most all low wattage systems do.
600 watts is not to play music screaming loud. It is needed to keep the lowest frequency Bass notes clean. And it could very well be using all the watts that are available, but only on the peaks. That's why the same music, playing at the same sound level, on 2 different amps, a 20 watt, and a 600 watt,....it will sound 10x better, especially on the heavy Bass notes.
Those low frequency notes that consume mega watts on the peaks, is consuming it for a small fraction of a second, therefore the system only needs to be fused for 20-50 watts of amp audio output.
