ok thats fine. The DAC i mentioned
http/lifehacker.com/five-best-digital-to-analog-converters-dacs-483393503 (the second one down) The O2 mayflower ODAC has different versions lets just say, as the above article says you can buy the standalone version without the AMP (pretty reasonable at that price) and do some DIY, or you can get the pre-build DAC without the AMP and pay more or you can get the whole DAC+AMP pre-built and pay a premium, at which point this is just un reasonable.
this is the DIY version http
/www.jdslabs.com/products/39/odac-objectivedac/
this is the DAC assembled version http
/www.jdslabs.com/products/46/standalone-odac/
this is the whole DAC+AMP thing http
/www.jdslabs.com/products/48/o2-odac-combo/
the first DAC+AMP mentioned in that article, the FIO Alpen E17 is pretty darn good for the price, and with this your headset/ headphones should sound very very good, however unless you're an audiophile getting an external DAC is not common with most people, however with that said the FIO will make stuff sound very very good with the HyperX or the DT-990s, The FIO DAC is very good indeed i have seem some reviews, however you can check reviews out yourself as well if you wish
as for the hyperX or the DT-990s, thing with audio is, the more you pay the better the audio experience is (usually)
The HyperX has a freq. response of 15-25,000hz while the Beyer has 5-35,000hz ,plus the DT-990s are open while the HyperX are closed, now theoretically that means that the Beyer will sound better and more natural than the HyperX however i have heard some amazing things about the HyperX in terms of audio when used with quality DACs, so cant really say anything there.
overall, you should indeed go with either the HyperX or the DT-990s and the FIYO DAC i talked about earlier
EDIT: also found this DAC+AMP
http/www.cambridgeaudio.com/products/dacmagic-xs-usb-dac-headphone-amp , very very nice with simpler connections, get this or the FIYO based on which ones cheaper really