You are. The HE-400 has less distortion than the HE-500 in the bass frequencies in the graph. Most notably 3rd harmonic.
As for the two posts above: you guys are thinking amplifier sense. Harmonic distortion and decay time can affect how deep a headphone can resolve into a recording. Surely you don't think that a pair of apple buds, nearing 100% thd+N, has as clean and black of a background as a pair of Sennhesier HD800.