'Headroom' refers to dynamic power available for musical peaks and crescendos. Dynamic power is what is on tap above and beyond RMS power ratings. The demands music places on your amp could easily require considerably more output for short bursts, often at much higher power levels than the garden variety receiver can typically provide. Your average receiver will have less headroom than a separate amp due to lesser internal power supply, and sharing of juice from the power supply to drive other things than the amplifier. As an example, I have a 50 watt/chan Yamaha stereo receiver (RX-396) and a 50 watt/chan NAD power amp (2100X). The dynamic power of the Yammie is about 80 watts@8ohms, 120 watts@4ohms. The dynamic power of the NAD is 150 watt@ 8ohms, 250 watts@4ohms. Despite their equivalent RMS ratings, the extra headroom of the NAD makes a huge difference, and results in a much more realistic, dynamic presentation.