I've not personally tried an O2 (nor ODAC), but I've only heard good things. I've been running Schiit's Magni/Modi Uber setup for a few years, which is no longer available because they revised the 3rd generation to include everything the Ubers had. Magni
has your gain switch and preamp outputs that you can send to speakers (if you want). Modi
accepts USB, coax, and optical inputs that you can switch between. $99 each plus cables should be well under your $300 budget.
I suppose that is technically a 2-piece system instead of an all-in-one, but they stack easily, and you can always upgrade parts separately. You could also drop down to a Fulla for a $99 all-in-one, but I have no experience with that.
The other thing I would consider is what headphones you'll be using in the future. V-MODA's stuff is great, but they don't have anything that requires a strong output. In fact, my go-to work headphones are the Crossfade 2 Wireless, driven by my iPhone. Just saying, there's not much more to get out of those drivers, and the drivers are the most important part of any audio setup.
On second thought, let's not go to TechReport. Tis a silly place.