Get a
decent soundcard if you haven't already got one, I recommend a
M-Audio Audiophile 2496. Now run your record or booth output from the mixer into the soundcard input, and run the soundcard's output into
a spare line input on your mixer. On your computer you
must mute the line in to prevent a feedback loop. I think this is the default state anyway, I never had to adjust mine when I set it up like this.
Now your decks will play into the computer for recording etc, and your computer will play through your KRKs via your mixer. Hope this helps!
No doubt some numpty will pipe up and say
"Run your mixer into one of the KRK's inputs, and run your PC into the other". Whatever you do,
don't do that! It will potentially overdrive and damage the outputs of your soundcard and mixer.