if you are looking for a set of full size cans, dont worry about what sound acrd you are getting. Keep everything bit perfect until it reaches the amplifier itself. What I mean is, get an amplifier that can take optical or coax input from the sound card, dont do ANY sound processing on the bit stream, do a direct LPCM passthrough to the amp. The amp is where you'll be spending the most of your money. at last as much as the headphones, if not more. An amplifier is extremely non-ideal, so you'll need to spend big bucks to get quality components that can properly amplify the signal. as for cans, dont cheap out and go for the medium pricey ones. you'll forever regret the purchase by thinking "man, just a little more and these would have been amazing". trust me. I just bought monster turbines for 58 bucks. If i had spent the full 180 retail price, i would have been rather pissed off that if I had been patient and saved up my money I could have easily gotten myself some custom IEM's or even a set of Earsonic SM3's (my next universal IEM will be these, i think)

I'm not sure why you would spend $200 on a sound card when you could buy an amp/DAC combo at that price. Why not buy the EF2 or a similar product now and omit the inferior soundcard?

No offense intended, but I think shrimants has amp and DAC mixed up. Also, the 595s reportedly have some durability issues, with the plastic headband cracking. There's nothing wrong with the K240s, why not stick with those? The HD555s and 595s sound unremarkable for the price, and Sennheiser's already released the HD558s and 598s. There's also the Shure SRH840s.

For an amp/DAC combo, just search "200 dac amp" in the search bar. But if you're too lazy, some options would be the EF2, Yulong DAH1, Audio-gd NFB-11, and Audinst HUD-MX1. All of those will probably outperform the sound card, and I can vouch for the Audinst begin a great product as I own one.