To be fair HE500 also has a treble peak. The difference is that HE400's treble peak is around 14kHz, while HE500's is around 10kHz. This causes HE500's treble to sound a bit more bright, and HE400's treble to sound a bit more tizzy (aka metallic as you say).
If you are looking for a Hifiman without any treble peak at all, HE5LE or HE6 are the ways to go.
























