Hello!
I also bought the 712 for peanuts on bax shops a month ago. Since then, i listened to them a solid 150 hours through a Roland Quad Capture dac into a Magni 2.
I'm a strong proponent of dsp to improve sound vs bit perfect playback, because i think one's hearing is part of the listening system and because of that if you want to hear flat and true to artist vision sound either you have perfect hearing or you use dsp processing. What importance does bit perfect playback if these bits don't sound as they should?
So on the dsp side i'm listening mostly flac through jriver, with audio upsampled to 192khz and feed into dmg equilibrium ( a high, high, high quality eq processor; i use an eq curve similar to the sonarworks one, but i add some bands of stereo equalization to fix some deficencies in my hearing that make me hear some high frequencies off center; i also run it in FIR mode at the maximum quality settings basically having the lows in minimum phase and mids and highs linear; this is the best advantage compared to plain sonarworks because it eliminates any pre-ringing and basically permits to eq the headphone without suffering strong transient smearing; the cons of this are that it's absolutely unusable for realtime studio work if you want to do that because the filter add a lot of latency and consume 20% cpu on my core i7, but for listening... hey, with quality we go lol) and redline monitor for crossfeed, then dithered down with tpdf. I find this gives me the best processing quality.
Aniway with this setup these headphones are killer, really balanced across the spectrum and with a soundstage not only large but also deep: more than once i had the impression of listening at monitors with the sound coming from in front of me. I would really like to hear a stock hd800 to see how they compare to the k712 corrected in this manner. Granted, the eq i use cost as much as the headphones, but i already had it so...
As for how they sound stock, well.. Good, but they are a bit to forward from 100 to 800 hz to be considered flat, so the bass even with it's 3db boost doesn't shine like it could. Also there are some nasty peaks on the highs that gives the impression of detail, until you eq the headphone the right way and notice that the cymbals ring and sound a little splashy. With eq, buttery smooth.
But since i was searching an headphones which didn't cost me a kidney but could deliver top of the line performance corrected, i'm super happy and for the price they go around now imho they are really a good bang for your bucks. Suffice to say years ago i bought the m50 for around the same price and they aren't comparable at all.