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!
You will likely need an interface device and/or something to provide power to the microphone. A simple XLR->USB cable will not work on it's own as it cannot provide enough power to the mic. Most XLR/USB interfaces will have the power switch you'll want/need for it to work correctly.
We personally use the Focusrite Scarlett 2i2. It supports two XLR inputs (for when we finally get a 2nd mic), and it has a headphone jack (though you'll likely need a for live monitor so you can hear your mic input without the 0.5 second delay a computer would create. If two inputs is one too many, there's also the Focusrite Scarlett Solo Compact. There are other interfaces out there, some even cheaper. You'll need to dig around.
While it may not seem like the biggest increase in quality an XLR mic does provide cleaner and less "tinny" sound overall. The reason USB mics are popular is the price point for entry is reasonable for most people because it's "buy one microphone of above average quality at $50-$100, you plug in and go". XLR's price point is higher because you'll need AT LEAST the microphone, XLR cables and the interface. If you can get started in XLR for under $100, congrats for finding good sales or (likely) sub-par equipment.
I personally love having XLR. It's expensive to be sure, but the quality of audio is clean and great. Not that you can't also get that out of a solid USB, but there's going to be ceiling and you'll need to jump in to XLR at some point if you want better audio quality.
At least that's how I feel.
I think the setup is definitely overkill, and you're missing out on an audio interface, which is IMHO the most important part of a setup like this. I wouldn't get the preamp at all - preamps (and especially preamps under ~$400) won't do too much to affect the sound, at least in a way that'll be noticeable when you use it for streaming. And you probably don't need a compressor either - they can be tricky to learn to use and even trickier to learn to use well. I would skip that too. If you ever need to compress something you've recorded, use Audacity. And compression shouldn't really matter if you're just streaming. And a mixer isn't really necessary for just one microphone.
But then, even if you got all of this nice equipment, everything would be ruined if you just ran it into your computer's mic jack. You should get a USB audio interface to connect your microphone and computer. I would recommend something small, like the Focusrite Scarlett 2i2. And this would remove the need for a preamp and a mixer, so all you should need then is the mic, the one long cable, and the interface. This isn't really overkill for streaming, and this will also allow you to record covers with decent quality as well.
I'm a bit confused how you made the music on your Soundcloud without even knowing that!
If you ever might use a microphone you should spend the extra money and get a Focusrite 2i2.
if you get the XLR you have to get a interface with phantom power such as the Focusrite Scarlett 2i2 USB Interface generally speaking its better to have analog recording than digital with an USB mic
Honestly, the Blue Yeti is actually a slightly better mic but it is USB and thus gives you no options to expand when you realize that recording guitar and vocals separately is so much better.
The XLR version of the Blue Yeti is twice the price, making it not worthwhile. Since you're already saying you want to record two things, you should discount the idea of the Yeti immediately.
What is your price range?
The best recommendation is the Focusrite 2i2.
This is easily worth the $50 more than the next option worth considering, the M-Track, even though the Focusrite 2i2 does not allow you to plug in an electric guitar without going through an amp or DI box and the M-Track does.
If you're wondering what the best use of your money is to get the sound you want to get, I can't tell you. If you're wondering if $50 for the AT2020 is a good investment, the answer is yes and you should buy two for stereo.
Look it up on YouTube, see if you like what you hear. I have 3 of them and like them but you can hear that they are rather "bright" almost in a tinny metallic way.