Going 3.5->AMP->headphones really isn't necessary; the DAC will be done by the soundcard and that will drive the overall audio quality. The amp is just going to make the output louder at that point.
Note the M50s really don't require an amp; they get plenty loud on their own. I just use my soundcards headphone amp for simplicity reasons (I'm using the 3.5mm output for other audio connections).
Also, as far as DACs go, remember there's more to quality then just SNR. Realtek chips output reasonably clean audio signals nowadays, but audio quality still suffers, be it from compression or just flat audio reproduction.