First impressions... I am using the Hex with Stax Lambda normal bias and a heavily modified SRM-727A (under Kevin Gilmore's instructions), which is basically a more modern version of the stock KGSS (with current source modification and the samet transistors as the KGSSHV, albeit not reaching the same voltages as the KGSSHV). It's not just the spritzer mod, it's a deeper modification. If I am not mistaken, Kevin will publish such instructions... otherwise, he has already allowed me to help in case anyone wanted to have some guidance to mod their SRM-727A. I have baptised my amp the KGSS-A.
Right now, this is the best setup I have ever heard and/or owned.
The Hex is much, much better than the Octave Mk2. Their main resemblance is in tonal intensity (or color saturation, if you prefer).
The Hex has much more deep bass power and dynamics, to the point that listening to the Lambda, you would never apply the word "light" to them. It's a continuous exhibition of power. Where the Octave has some bloom in the upper bass, the Hex is more linear. The Hex has similarly thick midrange to the Octave (which is where they share similar tonal saturation) but the Hex tends to be more forward in the upper midrange. The Hex manages to be as euphonic as the Octave, but much more balanced. There is still the same sense of romantic sound and fullness.
The biggest difference (if we try to "split" differences in departments) are in the treble. Where the Octave tends to be smooth, and lacks ultimate resolution, the Hex is super resolving. It offers a huge sense of layering, and super precise instrument placement. It has tons of information and nuances, and shows the micro-modulations of certain tunes (sorry, I wouldn't know how else I could call the effect, I am not very used to it with the other DAC's).
Anyway, the Hex is much better than the Octave Mk2. It's a no-compromise DAC... probably the first one I have encountered, albeit I need to listen to the AMR again, with this Stax setup.
I don't use all these superlatives without a reason, but it totally exceeded my expectations... well, now at least I can understand better what 6moons said by "Octave on caffeine". Albeit I'd have used "an Octave in need for exorcism".
Similarly to the new Yulong DA8, the Hex is an inspired and active "player". It's warmer, fuller and more complete than the Yulong DA8, and has superior spatial capabilities.
Listening to Okami soundtrack is particularly enjoyable.
With certain songs (I have yet to investigate if it depends on the track itself, it mainly happens with Television, or Queen), the screaming vocals can be too much. It needs to be evaluated how much of it depends on Stax Lambda, or if it depends on the Metrum Hex itself.
Soon, I should receive another Omega 2, so I will be able to test this behaviour.
Again, how perfect? It must be evaluated, in the long run, if being an "Octave on caffeine", as sixmoons said, will keep being enjoyable for every moment, but I might be paranoid at this point.
Thank you Tony, I've found your comments very focused, and I enjoyed your comparison with the DA8, which is another DAC I am currently evaluating for purchase (along with the Matrix X-Sabre, Audio-GD M7). The HEX is a lot more expensive though, doesn't do DSD, no HP amp (seen as a bonus), that makes the decision tougher.
From your description I think It would be a perfect match with the Senn HD800.