You're right...the Valhalla 2 is not by any means your traditional warm tube amp. Within the world of tube amps, it leans towards a more lean and clinical presentation. In my opinion, the HD600 and HD650 both benefit from this. The 650 especially. Though, again, I'm not tossing the Ember under the bus here. It does just fine with the HD lineup.
The Valhalla is an OTL tube amp and is designed specifically with high impedance phones like the HD6X0 in mind. But, if you decide to branch out in the future, especially into planars, then the Ember is a better option.
Don't dismiss the Ember. The HD600 sounds fantastic with it when paired with a 6N6P tube. Honestly, though, if you feel you're going to be using the 600 for a long time, I'd recommend the Valhalla 2. If you think your headphone stable might fluctuate a bit, then the Ember gives you better versatility.
The effects of daisy-chaining them is questionable. Some claim improvement. Some claim that it makes things worse. Some claim it does nothing at all. I'd recommend you skip the Wyrd and pick up a used Regen on the forums. Just make sure it's the "Amber" version.