Hmm. The HE-500 has more energy in the upper mids and lower treble compared to the HE-400. But the HE-400 do have a lot of mid/upper treble! Also the HE-500 is a bit thicker sounding in the bass. So the HE-500 could be the best bet if you found it too bright.
The HE-400 has a tonal balance very similar to the SR007 on my stat setup. The HE-500 tonal balance is really nice, just that the bass does seem less controlled than the HE-400 - which has cleaner and tighter bass.