The Mix pre 3 by Sound Devices is the under dog here compared to the Apollo X4 but it is by no means an inferior product. Both devices improve your audio quality across the board. A lot of these improvements come from the pre amps these 2 devices use, different but both are very quiet and professional.
Without getting into all the details a pre amp (preamplifier) is a device that amplifies or boost the signal coming from a microphone or musical instrument. So while boosting this electrical signal some pre amps have an inherit sound (a low hiss) that is often heard especially while wearing headphones. While this noise can be removed in post eliminating it will prior saves a lot of time and also makes your audio easier to adjust in post.