I have not had the Abyss at home for audition yet and have only heard the Abyss two times compared to the LCD 3F on a Moon 430 HAD. To me the Abyss do all the technicalities very very good and is for example much faster, clearer, more 3D then the LCD 3F and I would say also compered to the HE6, even if I haven’t done a direct compare. With the Moon 430 HAD at least I didn’t like the tone on the Abyss so much. I’m more for a bit of colorful tonality which both the HE6 and the LCD 3F have IMO.
To me, in that setup, the tonality was too neutral and dry and I want a more naturel sound with a touch of mid richness and sweet treble. I have been told that I might like the Abyss more with another amps like the VTL IT 85. I will definitely try the Abyss on some other gear, because what it does well, the technicalities, it does so very well that it can be worth to see if I also can get the tonality closer to my preference. I’m pretty sure that the Abyss will sound better to me in my rig than in the 430, but probably not all the way and I would need to get some other Amp than Master 9. A new amp and Abyss will probably be too expensive for me right now.
To sum it up I think it will depend on what sound sig you prefer and what aspects you hold as most important; fast, deep, clear, more resolving and very controlled bass, open and 3D sound - Abyss.
Sweet trebles, richer mids and more forgiving sound, probably – HE 6.