There's as much difference between different DACs as between different amplifiers, preamplifiers... a DAC is not just a chip. The elements around it (input chips, clock, power supply, and last but not least analogue output stage) all have an impact on the sound.
If you don't hear differences between two DACs, it may be that 1) those two DACs have very similar construction and components or 2) your system isn't resolving or 3) you're deaf.