The delta-sigma Gungnir is a good DAC overall, but relatively to Audio-GD NFB-28 & Matrix X-Sabre DAC, the delta-sigma Gungnir has a more noticeable "digititis" that sounds too aggressive/harsh, especially with HD800. The delta-sigma Gungnir does have a bit of smoothness that somewhat masks these deficiencies, but they’re still there when you compare it to the Matrix. The multibit Gungnir is a complete transformation of Gungnir. Nothing like my past multibit experience with the Valab DAC (TDA1543), which was fun and smooth at first but quickly became boring/flat.

• Gumby brings a nice step up from the Matrix. I wouldn't call it a night and day difference, but more like a step up to the next level. More fuller sound, easier to feel the space and presence of a live recording. Highs have less digititis to them, less fatigue, but more detail! Bass is more precise, especially with the Regen. Instruments sound more realistic. Even though there is less digititus, it is still there if you compare to analog sources (90s Sony tape deck, Music Hall MMF-5 turntable). I wonder if "leftover" digititis is coming from transport/USB issues or from the source mastering/ADC.
• Auralic Vega and Ayre QB9 are closer in overall sound to Matrix than Gumby. They're more refined and more resolving than Matrix, but still, overall sound is very close to the Matrix. This makes the value proposition of Vega or QB9 as quite poor, since the Matrix is nearly as good at a fraction of the price. Gumby is better than all three.
• Don't care for the lack of DSD since I often use EQ and crossfeed/headphone DSPs that have to be processed in the PCM domain. For my use case, DSD is worse than PCM since the roundtrip(s) between PCM and DSD would add distortions and high frequency noise that has to be filtered.
• DAC clicking on sample rate changes were a bit annoying so I minimized them by upsampling 44.1/88.2 to 176.4KHz and 48/96 to 192KHz (SoX VHQ minimum phase).
• It seems that Schiit USB Gen 2 is the weakest link of Gumby. Uptone Audio's Regen gives a very nice improvement to its overall performance, like Gumby on steroids. Gumby benefits from Regen more than the Matrix. If only someone would design a USB DAC interface that would make transport jitter/voodoo/etc irrelevant, as long as bits come in bitperfect. Reclock, regenerate, isolate, buffer, re-do everything in that schiity USB.

Headphones: HD800 with Toxic Cable Black Widow, Oppo PM-3
Amplifier: Bryston BHA-1
Transport: Windows 7, Foobar2000, MusicBee in WASAPI exclusive mode
Power: Furman IT Reference 7 balanced isolation transformer
USB: Uptone Audio Regen

Contrary to a common complaint, HD800's are not bright and do not *need* a tube amp/coloration. In most cases, the cause is the recording quality, which can be "tamed" by EQ.

Although HD800 has an expansive soundstage, you can bring it to another level by using a crossfeed like Redline Monitor. Overall presentation will be even more natural and easier to listen to.

Comparison rig:

Audio-GD NFB-28 via USB input, 0dB gain, balanced mode. Playing lossless music (with some high resolution tracks) on Windows 7 x64 and Foobar WASAPI.

Sound Quality:

- Soundstage of D5000 is noticably greater than Alpha Dog. Instrument separation is more clear on D5000s. ADs sound more closed than Denons (ADs have a greater effect on sound coming from inside your head). The AD soundstage is a bit congested and "fuzzy" with some tracks like piano solos. The soundstage of ADs improves very nicely with TB Isone. Great enhancement for those who play music on PC/Mac.

- ADs are definely more neutral than Denons, somewhat darker too, but not HD650 dark.

- ADs are less sibilant than D5000 (relatively speaking) and more forgiving to bad recordings with harsh highs. This works great for rock music.

- ADs have nice and smooth mids, instruments have "weight" in them, especially guitars (acoustic and electric). Better than Denon here.

- The bass of markl modded D5000 is more emphasized than default Alpha Dog. I found AD's bass just fine and did not mess with the screw adjustment. The fact that its possible to do that is amazing, wish more headphones would have these sort of built-in adjustments.

- ADs have less detail than D5000s

- ADs are less transparent than D5000s

Comfort:

I find the Denons more comfortable than ADs, but the difference is small. Both are very good. Neither are like the Audeze bricks. Denon seem to fit better for those of us with big heads . AD's leather pads are smoother and more luxurious to touch than the Angle pads I have on my Denons. AD's cable is quite microphonic.

Conclusion:

Overall, I preferred the Denons. IMHO, the main reason to get Alpha Dogs is for their great isolation, and better neutrality than Denon/Fostex equivalents.

Rating: A solid 7/10.

Some of the music used for comparison:
Sergei Rachmaninov - A Window In Time (TELARC)
The Body Acoustic - Bronxville (Chesky)
Antonio Forcione - Live! (Naim)
Giuliano Carmignola - Vivaldi - Le Quattro Stagioni
Dave Brubeck Quartet - Time Out (vinyl rip)
Enigma - Seven Lives Many Faces
Daft Punk - Random Access Memories (HDTracks)
Miles Davis - Kind of Blue - stereo (HDTracks)

The HD650s have been out for many years already and became a reference point to compare with other headphones. Although not the best headphones, they can still introduce you to the world of high-end sound, as long you provide them with proper amping.

I've auditioned these with DACmini for about a week. The overall sound is congested and seems to be missing something. Comparing to D5000, these have no fun factor at all. Add the fact that the comfort is much worse, the HE-500s had to go. As always, YMMV.

In short, better sound than build-in motherboard audio. The rest are cons.