CD players, DAC's, etc
Are not really always going to give you immdiate impression of hearing a big difference.. The only way to judge digital is more or less to listen for a more extended period of time to music you are fairly familure with, and really give them a go like a concert type playback, use several albums you are very expecting of certain things, and see how it emotionally engages you more than if you are just listening to another stereo or boom box with the same old sound.. Its hard to explain but like Vinyl you will hear a difference in impact, overall space, air, presence, naturalness
Many times its simply about Consitency, if a certain digital source can constantly please you with many recordings making a less flawed or better dynamic range, like how deep and impactful does the bass go, how smooth is the midrange and vocals, how good does the highs sound without getting harsh even at hi levels with raspier music, this will point you toward less distortion..
Many people make the mistake of testing digital sources at Low volumes, or ones that are just comfortable, you really need to drive the volume up even if its a little scary(not saying blow up your system) but rather let it play a little higher volume for a period of time to let things flow, see how much more control, and less distortion the digital source is making, and if you realize shes glued down really well, and you play your old source and it sounds more raw, muffled, or just sounds like your woofers are "Popping" making you think its not right than the digital is less engaging to you and you found a bit of a better digital source.. Its not an overnight experience sometimes, I believe in fact it can take a bit of a seasoned "Audiophile" to develop the taste of their own and pick out when something sounds right
Also fact is you really have to kinda accidentally experience it to realize it, or believe it sometimes, nobody here can put words on paper and make you hear the difference you seek.
Oh and don't forget first of all some people like a different digital sound than you, which is not right or wrong, so many suggestions will be made, however also your system, cables, and even the power from your walls can effect how good one player or the other will sound in the end.