It is true that shit in equals shit out. Doesnt matter how good a DAC is, if its receiving duff from the source, you will hear it.
The humble iPod has very good quality DACs and sounds excellent if you have copied your music over at reasonable bit rates.
Most computers are very noisy and again, an external DAC wont help. Here the old tower PC reigns supreme as it can run better and better sound cards whereas the laptop or even most tablets are ok to mediocre.
External sound cards can help but its an extra box and lead.
Home Theatre PC's were a short lived fashion item a few years ago and they still make a little bit of sense now - powerful graphics cards for high resolution displays, sound cards etc - if it wasnt for HDMI, smart TVs and Blu-Ray.
CDs are shit - simple fact, very few albums were mastered as good as they could have been. SACD was a dead end - its all on-line now and all the better for it.
Live recordings on to reel-to-reel are where its at, or live high resolution digital recordings if you want real Hi Fidelity otherwise its mediocre all the way!