Audio quest did a white paper on dac warmup:

http://www.audioquest.com/wp-content/uploads/2014/04/Phase-Noise-Jitter-Report-0317-14.pdf

I wouldn't think running signal through them would warm them up any faster, would it?

Signal means current--> current means heat.
Though in some cases other devs under the hood such as psu can be of greater importance in heat generation. Dunno if Yggs uses shunt  psu devices, some of my own units that do have so much heat transferred from psu-s that the other components heat production is dwarfed in comparison.

Valhalla 2 should work well with Gumby and not be a waste. Would be interesting to put Val 2 along with Mojo 2. The thing about Val 2 is that it works best with higher Z headphones like the HD650 or HD800. Also, Val 2 is typically not as warm as Mojo 2 - depends upon tubes.

Thanks for this info Marv.. I might be getting the HD800 again sometime and will probably grab a Valhalla 2 (again) to pair with it.  My first time around with the V2 was good.. but it didn't have the staying value I hoped it would have (though my DAC at the time probably didn't show what it was really capable of.. GO450).  Hearing that the V2 won't gimp the gumby is encouraging.

Some Gumby owners over at HF who've had it running for a good 100+ hrs. mentioned it lost some of its 'mellow character' (heard after the first 24 hrs of use) after 100+ hrs.  They mentioned it was more 'solid-state' sounding (or something like that) and less liquid/warm.. any truth to this?

Initial thoughts from me, having just listened to my first 4-5 songs with the MB Gungnir, feeding the original EC Zana Deux and HD-650's. I'm posting from my perspective, so please take all of these thoughts as completely subjective. I am also open to further discussion as well... I have a way of sometimes coming across as excited, so please feel free to call me out if you feel differently about any of my thoughts. No offense will be taken.

So the Yggy really blew me away with its presentation of details, and the very precise sounding treble response. For me, hearing an overall clarity as far as cymbals and percussion were concerned with the Yggy was a revelation. It removed a layer of garbage that always existed in digital music. I believe it was most evident in the treble, and I previously felt that vinyl was superior because it didn't seem to contain this garbage. Compression, crunch, sometimes a general glare or brightness... the difference between a hi-hat sounding natural, or a hi-hat coming off as something else. (A recording of a hi-hat that just wasn't the same as hearing one in person.)

Playing music on my computer, I was aware of the treble issue for myself, and it actually steered me towards tube gear. I found that tubes didn't remove the layer of garbage, but somehow brought out a bit more layers of sonic depth that helped to mask the issue with DACs. I also tried other DACs, and found that some tended to sound more laid-back, perhaps rolling off the treble, or not emphasizing it as much as more revealing DACs. The more detail oriented DACs (or more revealing ones) tended to really bring out the crunchy garbage, or glare. At best, they allowed me to understand/appreciate some of the lower level details I never heard in recordings... at worst, they just encouraged me to spend more money on vinyl.

The detail presentation of the Yggy is also a night/day difference from a lot of other DACs, though. The details are entirely present, in an audibly more correct, insightful way, while still retaining overall musicality. The Yggy is precise without sounding like it's doing something wrong to bring out the details. This was also huge for me. I heard nuances, spatial queues, and actual music, without distraction of wondering how. When I hear the Yggy, it sounds me like I am hearing what is there naturally; timbre, body and tonality without any kind of weird, exaggerated crap.

I also found the Yggy to do something with dynamic range that also kind of blew my mind. I never really understood how this could be done, but my understanding is that I'm hearing an expanded range of volume, revealing much more nuanced information than before. It's revealing nuances in how instruments are being played, how singers are forming their words, and it changes the presentation of the song in a way that captures atmosphere unbelievably well. I've heard tubes that can pull of a more "musical" sounding presentation, which is somewhat of a subjective term, but this is different. It's extra information in the form of an expanded range of volume, or dynamics... that is the way that I can best describe it.

Going to the MB Gungnir. Mine has been powered up for 170 hours, and I now have about an hour of personal listening...
I am hearing a very even keeled, smooth sound. The Zana Deux is hitting the 650's hard. I hadn't spent a lot of time with the Gungnir before the upgrade, but it seems to me that this was characteristic of the Gungnir before the upgrade? It seems to have more punch than I'm used to... yet it also sounds very refined and elegant at the same time. The 650's are really in another world for me as far as overall clarity and resolution is concerned. It's nice to hear this combination, and I think this is why my initial impression is very even keeled.

I need to spend more time listening to see how the treble compares with that of the Yggy , and also the details. I'm not entirely blown away the way the Yggy does, as far as cymbals/percussion are concerned so far... but this is by far the best source I've heard feeding the ZD, aside from the Yggy. I'm very impressed with the clarity. Bass is as tight and deep as I've heard on the 650's. Mids are elegant, but also full bodied without stealing the show. Female vocals are also satisfying, but I'm not quite getting the bone chilling insight the Yggy offered.

Time will tell... I am tempted to hook up the Wyrd tonight and see if it changes things.