The low THD OTL amplifiers like Woo and Schiit have been fine tuned with their circuit design, running the tubes in an optimal configuration providing clean amplification. Opposed to throwing enough voltage at a plate to make it conduct and give a "tubey" sound.
I think their primary factor for using them is because of the open case. You can't put an AC mains power supply into something like that unless the power supply or amplifier is totally enclosed. The power brick ticks all the boxes as they are already UL certified and don't need to provide any high voltage.
What did you find disappointing about the Valhalla 2?
In regards to the Valhalla 2 the amp lacked headroom and sounded a bit lackluster in body and was a bit too bright and lean in terms of tuning, it sounded quite bad with my T1.2, it was only decent on the DT 1990. It also had this weird grain/haze to the sound I've experienced with most Schiit amps I tried. It wasn't all bad, I thought it had good imaging and soundstage but I just did not like it's sound and it didn't change a whole lot when I changed tubes either. Was a disappointing match with my T1.2, did pair better with the DT 1990 due to the DT 1990's richer and warmer sound, but neither was all that enjoyable of a listen to me. While a hybrid the Lyr 2(which I also owned) was notably better and responded much better to tube rolling.
I'm not saying the Sunrise is a great amp just merely a good amp, it was just better than I expected and it responds better to tube rolling than the V2, I bought it out of curiosity and to tinker with it's output impedance settings, etc. The Valhalla 2 really isn't a good amp, while the design may seem superior on paper it really isn't in practice, not sure if it uses lackluster parts or what. When an amp(Valhalla 2) that can properly power a headphone(the T1.2) sounds worse than an amp(Sunrise III) that can't properly power the headphone regardless of the source, tubes, cabling, etc. then something is wrong with the design of that amp.