Looking for Headphone/DAC/Soundcard recommendation for Gaming/Music on PC.

[tridark1]

Hey, I currently own a PC with a MSI Z87-G45 motherboard which has a realtek ALC 1150 sound chip.  I'm replacing my SteelSeries Siberia v2 headset which worked for a while but I'm looking for a little upgrade.  Issues with my current setup:  headset clamps my ears and is generally unpleasant for longer periods of gaming, also the highs on these headphones over time really irritate me and is almost painful.  From what I've read so far the Sennheiser HD 598SE with 'warm' highs and over the ear muffs could be a good fit, but I'm looking for a recommendation on how to get the most out of them.  The Z87-G45 says it has an amp up to 600ohms, more than enough to drive the headset; but will I gain additional quality with a $50-100 DAC such as the FiiO E10K?  Will my headset make use of the realtek software if connected to the DAC via USB?  Should I consider a DAC with optical in instead?  Would a soundcard in the same price range be a better fit?
 
Thanks for all the help, I've read what I could from the guides but trying to piece it all together for my setup.