What is all in, when we talk price?
There's the first tier of things like the Fulla, DACPort slim, etc. ~$100 USB bus powered DAC/amp combos. Typically their biggest problems are lack of power to drive hard to drive cans and can have issues with noisy USB ports. They also tend to have synchronous DAC clocks, which can have issues with jitter if your computer's clock isn't the greatest.
Then there's the $200-300 range. These tend to be fairly similar to the $100 range combos, but have more power. May have slightly better DAC chips. Here lies the O2/ODAC, Modni/magni, etc
Then there's the $500-ish range, things like the m9XX, Chord Mojo, iFi IDSD, etc. Typically this range gets you a decent amount of power, ability to play ultra high sample rate music and DSD, a great DAC, USB isolation, asynchronus clocking, beefed up power supplies, etc. They may lack the highest end of pure power for very hard to drive headphones, and you typically won't be able to run a balanced configuration.
Then in the $1000+ range you're usually looking at a separate AMP + DAC, a similar DAC to the $500 range, a bit more power still, better build quality. Also you're still probably not going to be able to go balanced at this level.
Obviously things take off even further after that, but the features and similarities tend to be too multiple to really very easily sum up.
IMHO, the $200-300 and $1000-2000 tiers are kind of unnecessary for the HE400i, as the extra power that range tends to bring isn't really needed unless you play it *very* loud. I think with the HE400i, the first thing you need is a decent DAC, with somewhat low output impedance, and a moderate amount of power. Then the next big jump forward will come in the $500 range, with cleaner power supplies, class A amps and a better DAC. The final piece for the HE400i then probably comes when you can build a fully balanced setup for it in the $2000+ range. At minimum this is something like the Schiit GuMBy (1249) + Cavalli Liquid Carbon ($599 if you can find one) + balanced cable ($100).
The biggest benefit of the Fulla is that it gets the DAC away from the noisy internals of your computer and will lower the output impedance, giving a bit better damping than your average computer motherboard. The O2/ODAC would add more power that you don't really need, plus the aforementioned "digital harshness" of its THD profile. Until you are ready to truly upgrade the DAC and get into a class type design in the $500 range, I'd just stick to the simplest combo DAC/amp that will get the sound out of your computer.
