So at the moment I have an Astro A50 for gaming and everything else (ie youtube, ect) and a Fiio X5 with an old pair of sennheisers (54ohm I believe) for listening to audio through foobar 2000.
But I want to upgrade my set up soon. I'm looking at getting a set of Beyerdynamic DT-990 @ 250 Ohm (or whatever I decide on from Beyerdynamic). I'm looking at the O2+ODAC as my audio device to power my new set of cans since its designed to be more neutral sounding and not effect the audio of whatever I'm playing. But recently I learned about the Magni / Modi 2 Amp/DAC which seems cheaper and offer some inputs the O2 doesn't have.
I intend on using USB audio (or optical) for PC, RCA for wii U, and optical for my ps3/xbox one. Anyone used any of those products and can tell me what would be easier for PC (ie USB or optical and sounds the best for gaming).
Aside from that, if you don't have any issues with your onboard audio, don't worry about a dac and amp. The improvement will be marginal if best, and might be nonexistent.
As for which DAC/amp you get (assuming you actually need them) pretty much all DACs should sound pretty much the same. The guys on audio forums who use flowery language to describe why DAC X sound better than DAC Y are pretty much just using that as a justification to keep spending money on an expensive hobby. The same logic applies for an amp (unless you specifically want something that will color (read: distort; change) the sound of your audio.
All inputs should sound the same unless there's something wrong with the components, and whatever sounds best for music/youtube/whatever else you want will sound the best for gaming.
My overall recommendation would be to check out some more of the FiiO lineup. They're cheap, measure pretty well, and they have lots of variety. I'm sure you'll find something that meets your needs. However, if you want to be sure that your audio sounds as best as it can, the O2/ODAC are pretty hard to beat. I hear that the low-end Schiit products sound pretty similar (as they should) but I don't appreciate some of the stuff that Schiit has done in the past, so I don't really like to recommend them too much.
If you have an exotic cans, like Stax SR-009 or HiFiMAN HE-1000 or Audeze LCD-3 or Sennheiser HD 800, then amp pairing matters a lot more! Otherwise, an O2+ODAC like the ones from Mayflower or JDS Labs will do the job more than adequately. DT-990 is far from exotic.