I never understood the vocal vs instrumental forward backwards thing. We do know that both vocals and the majority of instruments' sound occupy the mids? Shouldn't the larger treble spike on the HE-400 compared to the HE-500 give the HE-400 more vocal presence, since people like to complain about it having too much sibilance? Sibilance is a vocal trait, no?
"The mids" is a huge overgeneralization though, our human hearing is honed in on midrange frequencies so it distinguishes between details in the midrange much more succinctly than in the bass or treble, so just saying A and B both occupy the mids doesn't mean much.
A big part of human vocals is upper midrange~lower treble, which is much less represented in instrumental sounds except maybe stringed instruments, or digital instruments like electric guitars.
Other instruments more bracket the bass~lower mids (all kinds of drums, bass guitar, and beat-producing instruments) and treble (snares, cymbals, etc etc), these are unaffected by HE400's colouration so they blast at full force. So in the end the vocals get pushed back in music that has a whole ensemble of instruments.