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!
Hey, I can actually make a solid recommendation! This must be my lucky week for posting here.
I've got a pair of AKG K240 mkII's I love very much. I got them at a good deal on Amazon when I ordered the old version and they sent me the new version either by accident or through trying to phase out the old version. In short, they sound fantastic. They're pretty flat in terms of frequency response, but being over-ear they still have pretty deep-sounding bass. I've found them really nice for recording and mixing.
The only downside I can think of is they can get pretty uncomfortable over long periods of time, but this is definitely a personal thing, as I have pretty weird-shaped ears that have bits of flesh sticking out everywhere.
If you can't afford the the $110ish pricetag (I couldn't when I got them), it appears Amazon still sells the original version for around $70. They are (according to what I've read) almost exactly the same headphones in terms of drivers/construction, just with less padding and fewer accessories (the mkII's come with a second replaceable cable, a different set of ear pads, and an 1/8" to 1/4" adapter). I have no doubt these would also be an excellent investment if they in fact are the same headphones as the ones I own. Here are the originals.
Hope this helps!
Depends where you're at as to the options you may have available, but in the US market for example, the Fidelio L2 is another great option. There's also the Takstar Pro82 for a closed-back. And the AKG K240 / mk.II is also an option.
I'd recommend these instead, but if you want wireless ones go for those I guess
Edit: grammar