You apprently didn't read my post... I had both the DT 990 and the HE 400, and the he 400 did not out perform the dt 990 out of my oDac and M Stage, it took about $800 in gear for the HE 400 to reallly outshine the DT 990, when I got my NFb 10ES2 and a balanced cable for the HE 400, only then running fully balanced did it out perform the Dt 990, other wise out of my M Stage and oDac which cost me liek $300 total, they perfomed very much on par. THe differance being sonic presentation. Better mids on the dt 990 better sub bass on the HE 400
I have actually listened to higher end headphones like the HE 400 [for months as well mind you], so unless you have heard a $400 don't make assumptions about them. Headphones in the upper teir of mid fi, such as the W1000x, the HE 400 they need high end amps and dacs to perform at the expected level. Without higher performance Dacs and amps to feed them they are sonically on par with low model Mid fi gear like the UE 6000 and the DT 990.
Now going to the UE 6000 whats makes it SO good is it's sound! But the UE 6000 does NOT scale well. So for the $100 it is an EXCELLENT headphone, it doesn't need a high end dac or amp. So an E17 would be a good step above on board sound. And it would be more than enough for an UE6000. I find it best to get your self some intro gear. A simple Dac and amp. Then once you have that, then work into headphones. Don't feed a $400 headphone out of your On Board pc audio that's a waste. Spend $200 and at least get a Schiit Modi and Magni [or Vali now] and then from there with that intro dac and amp, you can run all the way up to an HD 800. THere are guys enjoying the HD 800 out of the Modi and Vali, so the intro dacs and amps are a wise invest when you have nothing. The E17 is a good palce to start if your really on a budgetm from there I recommend keeping the E17 for portable use [at school in the library at your friends hours ect...] and investing in about $300 to get a Dac and Amp. Such as the mentioned Schiit Stack. Those intro units can last you for a while! I know guys Running a $1000 Beyerdynamic T1 out of a oDac and M Stage, and there is nothing wrong with that, but investing in headphones should come after you at LEAST has some intro gear, a quality dac and amp [or a single combo unit] as those compents make a noticeable differeance over what ever Basic Onboard configuration your have on your PC.
With headphones, you get what you put into. Feed your headphones crap the will out put crap. Again the E17 is a great starting point. It will nicely run almost all of the $200 headphones. When you can upgrade to something on par with a Schiit Modi/Magi[Or Vali] or even something like the
Audio GD NFb 15.32 Which is a dac/Headphone amp for $235 [about $300 total with shipping] and with that intro dac and amp, your good to start trying the nicer high end headphones.