The M3K is a digital audio player ( DAP ). It holds your music and has a nice little DAC integrated. The E10K is a DAC and headphone amp. So they're different things. I have the original M3 ( non K ) and it's capable of driving my lower impedance headphones and IEMs and sounds very good especially considering the somewhat low price. It should drive your M50X easily.
I'd be more worried about whether you like the M50X. It's a fine headphone for studio mixing but I don't find it very enjoyable or musical for pleasure listening.