DankkDankk, this was my same scenario several months ago. This DAC/Amp will make the M50's sing. You should hear a noticeable difference. I have spent many an evening experiencing the wonder that good "cans" deliver with a DAC/Amp - specifically, this model. This a great beginning DAC/Amp for the budget audiophile.
DankkDannk, keep it simple for the moment. If the DAC/Amp is to be used with a PC, then use the USB. This particular one will perform marvelously with that selection of input. There are too many arguments, from too many audio-geniuses to count, regarding which is input is best. When you make your next step up in equipment, research this again. Otherwise, the best advice to remember is that if you begin with crap, you end with crap. Invest in the best you can afford and have fun. Here is a link to a great resource of knowledge and opinion: http://www.head-fi.org/
DankkGelatinous_Cube said it well. However, since you have both options to connect the X1, try them out yourself. If your Mac/PC has optical out (most do) then you just buy an optical cable for $5 and compare yourself to the USB connection.... Oh the fun of fiddling around with gear...
Gelatinous_CubeHey! I just got the Aune X1 Pro and I was wondering if you would be able to answer a couple of questions that I have regarding it. I plan to use the X1 for both my headphones and speakers. Would it work for both if they are both connected and I would just need to change the output? If I connect the Line Out from the X1 to my speakers, will I be able to control the volume of the speakers? Thanks!