@Bomble, my question to you is.....does your present setup get loud enough for your M50X? If so, don't waste your money on amps as they simply make sound louder.....not necessarily better. I assume you're using your mobo audio, if the DAC in it is capable of decoding all the audio codecs (FLAC, AIFF, MP3, WAV, and DSD, last being capable only by external DACs AFAIK), then don't bother buying anything else.
FYI, external DAC's can be of two types, dedicated DAC or combo. The former is capable of decoding digital audio data into electrical signals or continuously variable voltage (analog) (or some malarkey like that). The latter DAC combos have an amp section built into it to power headphones.