The lack of euphony, as in 'body', 'soul' or 'life', etc. is something the Sabre DACs don't really seem to enhance. They have a light, airy tonality that enhances detail, soundstage and resolution. They won't add much musicality to the sound. From my understanding, that's what tube gear or warm DACs/amps should add.
You already have a tube amp so you should be covered there, though. The Gungnir really is very different from the Sabre DACs I've heard [NFB-7 and Compass2, both Audio-gd]. I'll be able to comment on the X12 when I get it.
The Gungnir is a musical piece of gear, but it has its sacrifices, such as those I pointed out before [resolution, details]. It could fit very well with the HD800, albeit I have not heard it. I own the HE-560 and I preferred the Sabre pairing with them and my 'warmer' SA31SE. The Gungnir was just too musical and smooth for its own sake and IMO took away the airy qualities that make the HE-560 so believable. But people like Sonic Defender really like the Gungnir-HE-560 pairing... Lots will depend on what one is accustomed to and what one's preferences are and what he listens to