Here's my impressions thus far on the Fulla 2, compared to my onboard sound and Denon AVR-X2000 HP port with HD598's. I've done all my testing with Blu-Ray movies and PC games, since that is my primary usage. I ordered a set of DT990's to try with the F2, but based on responses in this thread i'm not hopeful. I'll update if I'm pleasantly surprised.
My initial impression of the F2 was that there was an extra level of clarity over both my mobo sound and receiver in comparison. The more I listen though, the extra detail was just due to lower output from the F2 on the lower end. The onboard card on my mobo doesn't have a great amp, and uses the Realtek ALC898 codec. I prefer the onboard sound to that of the F2 due to it being much fuller down low, while still maintaining detail on the high end. If I had to make a choice between the two, I would still choose the F2 as it has a higher volume threshold. There are times when I would like a little more volume compared to what my onboard can provide.
Plugged into the receiver is a similar situation to the above. The sound is fuller than the F2, and I have plenty of volume headroom. So the receiver wins hands down here for my use. I don't know what the impedance of the HP port on my receiver is, but I assumed it wasn't really intended for frequent HP use. I would have bet money that the F2 was going to come on top of this comparison, but that wasn't the case.
So where does that leave me with the Fulla? I'm torn to be honest. It doesn't sound as good as my other options in the physical space it would be used at home, so I won't use it there. I would consider it as a mobile option for travel and work, but the USB ports are so tight I don't want to use it unless i'm plugging it in and leaving it in place. Like tight enough it feels like I might snap the end of the cable off while plugging in. I've seen where there is potentially a way to adjust those myself, but i'm not keen on doing that. I think the F2 will be a great option for some people, just not me. I'd have to put extra feet on the bottom of this thing to balance it also. It is incredibly annoying to have it rocking back and forth.
I've heard all the great things about Shiit as a company and this is my first purchase from them. I can say I was impressed by their representation early in the thread regarding their recognition of the iOS issues and its resolution. On the flip side, their more recent representation in this thread is horrendous in my opinion.