I have a large experience with CD players and DACs, dating back to at least 1996, when I was in Edinburgh for a year and got infected with the persevering, and usually lifelong, audiophile disease - from magazines like What Hi-Fi and Hi-Fi Choice A good DAC should in principle improve the SQ of a CD. But the CD must have a good transport section and the cabling must be good. One of the snags is here. The best connection is 75-Ohm BNC. But BNC inputs and outputs are rare. Cambridge Audio used to mount BNC inputs and outputs on their CDs and DACs. Not sure if they still do. BNC is 75-Ohm standard, whereas a 75-Ohm cinch connector is a rarity (WBT do produce 75-Ohm cinch connectors, some of them not overly expensive). Of course, the analogue section, after the DAC, must also be good. Otherwise although there may be a clear improvement in the CD-DAC upgrade, it will reveal itself in ... somebody else's system
Sources: Advance Acoustic MCD-403 & MDA-503, Pure Sound A-8000, HDD + old notebook + M2Tech Evo Power Supply + M2Tech Evo Clock High End Word-Clock-Generator + M2Tech HiFace Evo 24Bit/192kHz USB-Interface