So right off the bat the KRK-8400's will give u a skewed perspective on your music. As shown in this graph the comparison between the two http://graphs.headphone.com/graphCompare.php?graphType=0&graphID[]=963&graphID[]=2961&scale=30
The KRK-8400's are closed back headphones. They hype up the bass and boost the mids vs the dt880's do not and will stay true giving you a neutral sound to mix on. There's nothing worse then being romanticized and fooled into thinking the music sounds good just because the headphones make it sound good.
What I Suggest is you get a pair of AKG 240's or Sony MDR 7506 both will run you about 100$ that way you can get a good audio interface like the focusrite 2i2 and the amp.
Links for the headphones that I'd recommend.
Sony MDR 7506 $100 http://www.amazon.com/Sony-MDR7506-Professional-Diaphragm-Headphone/dp/B000AJIF4E
Focusrite 2i2 $150 http://www.amazon.com/Focusrite-2i2-USB-Recording-Interface/dp/B005OZE9SA/ref=sr_1_1?s=musical-instruments&ie=UTF8&qid=1431837568&sr=1-1&keywords=focusrite+2i2
Keep in mind that you can get the audiointerface for about $100 on craigslist I'd just buy it of craigslist and get the amp and headphones from guitar center Plus price match everything _^
And yeah you did choose some really sick headphones to mix on to begin with. The dt880's were the first pair of headphones I bought and I did a ton of research before buying them. But if you want the whole enchilada the headphones the audio interface and the amp I'd choose this avenue.
Please don't choose the krk-8400's cause you might regret since the translation might not work well because they are hyped.
Good luck!