I received my Fulla 2 yesterday, and have been putting it through it's paces both last night and this morning.
I tested the Fulla 2 with both my MacBook Air and Windows 10 PC, both connected easily and neither required the addition of a secondary power supply. I also hooked up a pair of Kanto YU3s via the line out and had zero issues. I prefer line out to the YU3s since they have a remote for volume control, but I also verified that the pre-out works as expected.
For comparison purposes I used three different headphones: Sennheiser HD598Cs, Philips Fidelio X2 and Sennheiser HD 650, and directly a/b'd the Fulla with an SMSL 793II, using Tidal Hifi.
With the X2s and the 598Cs, I much prefered the sound of the Fulla 2. It's not night and day, but the bass response seemed more controlled (especiall with the X2s) and treble was generally smoother. I really, really like the way the 598Cs sound with the Fulla 2... it might become my go to mobile setup moving forward.
The HD650s were a bit of a different story. Now, before everyone says, the Fulla 2 isn't meant to power the 300 ohm HD650s, I specifically asked Schiit and they assured me the pairing would work out well. While, I don't disagree, the HD650s sound just fine with the Fulla 2, and do get extremely loud, the HD650s definitely do benefit from more power. I've noticed a slight reduction in tightness of bass, and overall expansiveness of HD650s compared to powering them with an Aune X1s. What's most interesting, is, at least to my ears, the Fulla 2 and the SMSL SD793 sound nearly identical with the 650s. It may have been the tracks I selected, but the 'noticeable' improvement I heard in the X2s and the 598Cs switching from the 793 to the Fulla 2, were missing with the HD650s.
Having said all of that I'm definitely keeping the Fulla 2. I might add an Magni 2 at some point, but honestly, although I know I can get more out of my 650s with better amplification, I'm extremely happy with the Fulla 2, especially at the $99 price point.