headroom = maybe its because the harder you run an amp the more likely it is to 'clip' the signal = distortion, a 150 wrms amp running 'full tilt' IS going to clip the signal, so if you have 150wrms speakers and you run a 300 wrms amp you can easily run 150 wrms to your speakers with no 'clipping' = 'distortion', the amp will be well within its ability to reveal all the majik of the 150 wrms speakers = headroom.
fyi