Low frequencies are more power hungry than high frequencies for a number of reasons. The peak demands are much greater in the low and midrange frequencies (below 300hz) because simply there is more content in the source material than content at 1000hz, 3000hz, 5000hz 10000hz etc. The most important aspect to look at is peak content in the source material below 200hz. When you look at impedance graphs of speakers its also very common this is where impedance minimums range (they can even dip in the 2-3 ohm range with some speakers. So running the speakers large this is where the majority of the current demand will be coming from.
