My initial impression corresponded to your first impression ("Bassy, dull, dark"). But luckily, it clears up after a few hours of play-in time - bass gets more refined, sound clears up. At this point, you might have some issues with a somewhat harsh treble, but this also clears up after a few more hours, when the mids finally arrive at their well-earned centered place at the sound party ;)
I´m a bit confused about people who sell them after barely one day for sound-reason. They should have given them one to three more days. But - if the initial sound had remained even after a few listening-hours I would have gone the same way. Luckily, it went another way.
One more information about sourcing it from a smartphone:
Yesterday, I listened to them directly via my iPhone 6 (instead of my DAC/AMP Onkyo HA-200). Yes, you can drive them, yes it gets loud enough, but I had to put the volume to 100% to manage a level that barely fits my convenience, relax volume level. But no reserves, bass area was WAY beyond my amped setup, mid and treble were very silky, well balanced. BUT it does clearly not manage to unlock the whole detail and richness, you get with a decent amp setup. Does not need to be a "ginormous" desktop setup - but you can clearly hear a big positive step in listening experience. Amping it, turns the K7XX from "nice, but ordinary, somewhat light sounding" to "OMG, I´m even hearing the earrings of angels resonating in mid of a bombastic, huge sound landscape of a heavenly choir."