I went from an Asus sonar DG ($27) powering my T50rps, to a Schiit Uber stack. Modi and mangi 2 Ubers. Running through optical. When I first started listening to them I was under welmed. $300 and I hardly notice it. Even with a pair of orthrodynamics.
But now my goodness, I understand why they are so good, flipping back to my sound card, I notice a huge difference. That stack and those t50rps add up yes, but value per dollar to me, is more then gold. Just took a while for me to pick up on it
Now that being said.
1: files. A low end mp3, will sound like a low end mp3. Period.
2: USB: Jason himself from schiit audio said usb is messy, which is why they sell Wyrd. Especially in a laptop outputs are messy. No 100% saying that's the problem, but it could be a factor (maybe)
3: RF signals. The cord to your cans can act as an antenna. (Litterally what ipod use to drive the fm radio). It's rare, but some users report interference.
4: Cans, those are decent cans, so the DAC should make a bit of difference.
Or you haven't found the difference yet, I know that sounds funny, but once you hear the difference you'll notice it. Its like you finally notice the one tiny dust bubble under your screen protector, now you can't unsee it.