I have had mine a few days now. So my opinions are still based on early listening with about 30 hours burn in. The reviewers I respect say they won't shine until 250-300 hrs burn in.
I'm an audiophile. I want to hear the musicians as if they were right in front of me, or as if I were in the recording studio. So, any equipment I buy I'm critical about. However on saying that I have only ever owned high quality components and have never tried cans, so please take my opinions based on the fact I have very little experience with headphones. My first foray into headphones is admittedly more on the budget side of things, but my comments are based on my normal equipment I use daily. Perhaps not a fair comparison, but thats how I roll! Before you ask B&0, B&W and Niam. (I grew up in the UK, but live in the US, so european HIFI is more to my tatste)
I have paired my K7XX with an SMSL M8 and a Schiit Vali 2 so you know my sound profile. All tracks I am listening to are at least 192/24. Out of the box bass is strong, mid range is meh and treble is bright and grainy, however sound stage was wide and addictive as you would expect. On trhe whole I was quite disappointed in them after everything I had read. So I decided to leave them playing for two days without listening to them.
Woah, what a difference! Bass is now more controlled and is never boomy. Treble has lost some of its graininess but isn't fully there yet. The thing that bothered me most was the sub-parr mid range . Its becoming more defined, but isn't where I would like it yet, but based on the improvement I am already hearing with a fraction of the burn in time, I'm encouraged. I will say these headphones sound slightly dark, which is something I like, but you may not. Obviously neutral is best, but I personally find a darker sound more seductive and makes me say, yes I'll do that after one more track!
I will say that, crappy recordings sound pretty good for what they are, but the K7xx's will highlight their shortcomings. Yeah I tried a couple of bad MP3's. Like many of you, I have a track I listen to when I test equipment out of the box...mine is Prince, Purple Rain, as it hits pretty much every octave I can hear (don't tell anyone, but I don't even like this track, but it always is unforgiving of subpar equipment). I have a high quality FLAC and it sounded terrible! It was the first track I played. I felt as if I had made a mistake buying these headphones. Then I skipped through my list of tracks and tried Hotel California. It sounded so wonderful I made everyone in my household listen! Bear in mind this was straight out of the box.
So, I, currently have mixed feelings on these headphones, the brightness and graininess is diminishing, the mid-range is becoming more articulate and bass is tight and has appropriate weight where required. I'm impatient and can't wait to hear them when they have fully burned in. I'll post again after the burn in period if it will help anyone.
As an aside, I can't recommend the SMSL M8 enough. It nearly matches my main system DAC at a tiny fraction of the price. It may not be a full 9018, but the difference is negligible. I never see reviews on this DAC and it always frustrates me. Oh and the Vali 2 is surprisingly neutral. If it has the tube/valve warmness I am yet to hear it, but there again I am using the stock tube and haven't rolled it yet.
I'm 42 and can't hear as much as I used to, so take that into account. I hope this helps someone out there.