When my laptop is connected to my mixer, there is a hum/interference through my speakers which doesn't vary with fader/volume control.
It almost sounds like the mixer is picking up the noise from the CPU / processor or it could be an earth loop.
Do you know how I can solve this?
When dealing with mains hum there are many techniques to try to eliminate this, such as keeping unbalanced cables as short as possible,
and certainly not coiling excess cable length (use a figure-8 system to avoid turning the cable into a transformer).
Keep audio cables and mains cables as far apart as possible, crossing at right-angles where they have to meet.
However, more significantly in your case, does the noise disappear when you remove the mains cable from your laptop, running it only on battery?
If so, then yes, your problem is very likely a ground loop in the system. You may not experience this on other mixers, but because the Xone mixer has a metal chassis, not plastic, it is essential that the mixer be grounded via the mains cable.
Under no circumstances should you consider removing the green and yellow mains earth wire from the mains plug to the Xone, as if you do so, then should a fault develop in the mixer's power supply there is a risk that the chassis of the mixer could become live; a potentially lethal fault.
A much better way to address this problem would be to find a way of isolating the ground connection in the audio cabling.
This can be difficult to achieve on RCA phono cables with moulded plugs as the connection terminals can be hard to get to without cutting off the RCA phono plugs completely and replacing them with solder-on connections which you wire up yourself.
If you can do this, then simply try leaving off the connection to the ring tab at one end of the cable (the cable will still be screened by the connection at the other end, so the length of the cable will be shielded).
An alternative way of resolving this would be to use a Ground Loop Isolator