Hello,
I don't think it's worth it to buy a too much expensive headphone for ambient music.
I'm not particularly a fan of the hd595, but if there's one area it excels, it's ambient music.
But you mentioned dark ambient, so probably you'd want more sub bass than the hd595 is able (for the scary effect, you have with some bass drones).
I think here are the quality for the headphone you are looking for:
- nice sub bass
- great soundstage
- anything but bright (I mean no emphasis of the highs).
- not excessively analytical, preferably something "smooth".
- some euphony , if you listen to stuff like stars of the lid (i.e even when playing a single note for long, it sounds pleasant).
The hd595 offers all of this except the sub bass.
Listening to merbow would be painful on a detailled and bright headphone , which seems the case of the k701/ q701.
The sonic difference between the k701, and the q701 is most likely due to the presence of the logo that partially hinder the air flow.