[Rikifi]

I just bought some krk kns 6400.. Great sounding btw.
Sound pretty good through everything I've tried, but when I plug them into my soundcraft gigrac 300 (I play keyboard and this is my powered mixer) and out the headphones it sound horrible.
Manual says headphone out is 18dbu.

I really want to buy a proper headphone amp, why are they distorting and sounding blown through my gigrac and will they be like the through ant amp I get, if so I'll return them.

 

[NA Blur]

First what do you mean "normal" volume?  Please describe exactly how you have your setup.
 
For example:
 
PC set at 50% volume --> gigrac 300 at 80% volume --> headphones
 
It could be a number of things ranging from impedance to clipping to a bad track, to you are actually hearing what you are supposed to hear.

 

[Rikifi]

Tv headphone out is at 50%,gigrac at about 70% gets me enough volume but it sound distorted and sounds like some clipping is happening.. Put some 200 ohm headphones in and it fine.

 

[NA Blur]

The output impedance of the gigrac may be too high causing distortion ( poor damping ), but you may want to try turning the Tv output to 90% and turnding down the gigrac.  That may help prevent clipping which also causes distortion.