Funnily enough, I found exactly the opposite. To me the HE-500 bass has more strength, punch, which makes it a better can for EDM, Rap and other bass-oriented genres. The HE-400 on the other hand has a somewhat "cleaner" bass, which can be explained by the lower distortion on measurements. Also, I found the HE-500 to be more forgiving on poorer records, due to the more recessed treble compared to HE-400.
I still find HE400s with the stronger bass punch, it is more tactile than HE500's bass because of lower distortion values.
HE500s have bigger-sounding and more "complete" bass, however, which I think should be distinguished from a stronger-hitting bass. The 500s extend lower and seem to have higher resolution / texture in the low frequencies despite the higher distortion relative to 400s.