I swear you're in a honeymoon period. Maybe from all the exposure to deprived midrange.
I had the Q701 for a good period, and it has great mids.
But you're somewhat right, as I still am a V-shaped person overall.
As for the HE-4 vs HE400... I really need to get my hands on the HE-4 again to do a legit comparison. It's been a long while since I've owned it, and all I remember is that in comparison (from memory), the HE-4 has distant mids, and the treble is more sparkly and less refined. That is somewhat my preference for music anyways, but the HE400's treble has the perfect amount of treble (still sparkly), and the mids are something of a revelation for me. The bass is similar, but the HE-4's bass is slightly loose, which makes it more 'fun' sounding. I can't be 100% sure about actual bass quantity, but from memory, the bass wasn't lacking even for me, but still less than the DT990 (which is one of my bassy cans in terms of bass quantity and quality).
In order for the HE400 to be nearly perfect, I'd have preferred more bass presence, but that by no means it's lacking. The HE400's bass is rich, and strong when necessary. I just have a preference for more emphasis (being used to the D7000s and all). The HE-4 could use slightly less treble. Other than the slightl tonal shift from the HE-4s slightly brighter than the HE400's slightly warm tone (Rev. 2), I think they share a lot of similarities, and you quickly notice that it's the Hifiman planar sound.
Oh yes, the HE400's and HE-4s driving requirements are night and day. While the HE400 certainly scales with amping, it doesn't require much to sound great and loud. The HE-4 is very, VERY strict in terms of amping, and REALLY needs a ton of power. You can forget about portable amps. Some mention speaker taps also being great. I believe the HE-4 cable with just that, but it was DiY. I'd have wished it came with a cable with an interconnect that allowed the option for speaker taps or standard headphone jacks. XLR it's called?
Seriously, I NEED another HE-4 to do a side-by-side. If anyone is willing to send it out to me, that would be LUUUUUURVELY.