So you came over here looking for ways to improve the sound of your system but after looking around a bit you find that it's kind of easy to become obsessed with this stuff. You're noticing that audiophilia is a disease, and a very expensive one. My advice is to set some ground rules for yourself and stick to them as long as you can. Here are mine:
1) Stick to 256k mp3s. These are the files you've already got. They can sound awesome. Resist the urge to even try some hidef. You will receive offers to try out hidef files for free. Ignore them.
2) Stick to $200 headphones. There are plenty of great $200 headphones. I want to try them all. That's another problem.
3) Since I use a Surface 2, I'm sticking to portable, USB powered DAC-Amps, also under $200. This is an interesting segment of the market with great new products arriving every 6 months. Possibly yet another problem.
I know it's tough to stick to these limits. About a week ago I became obsessed with getting some HD650s and a Bottlehead Crack. Fortunately, a quick look at my credit card balance brought me back from the edge.
Good luck and good (enough) listening.