DACs are basically converters from Digital to Analog. To improve audio quality, need to take note of a quite a number of things.
1st: Headphones/Earphones
2nd: DACs (mostly external)
3rd: Audio files (lossless)
4th: Cables used connecting them together
For headphones, DACs & Cables, you gotta try it yourself to see if you like what you're hearing.
For audio, recommended that is rip from CD.
But if you want something simple, nothing complicated, since you already own the M50X, change the audio files you're listening to and get a better DAC.