Charlie Hansen:

All of this can be boiled down to a simple phrase. “All of the problems with digital are analog problems.”

This is the primary reason that digital audio has taken so many decades to come close to the sound of analog.

Gordon Rankin:

But since digital audio is a streaming system, the timing of the bits is critical. If the bit changes to the correct state but at the wrong time, this is equivalent to changing to the wrong level at the correct time.

Another area to tackle is what is referred to as signal integrity. The signal leaves a transistor or IC chip and it has to make its way across the PC board, component-by-component so that the signal is degraded as little as possible. When you are talking about what makes one transport a “good sounding” one, again we are talking about treating so-called “digital” products as very high speed analog circuits. The clock frequencies in these units are typically between 10 and 100 MHz. When considering a square wave, a convenient rule of thumb is that the bandwidth must extend in both direction (higher and lower frequencies) by a factor of at least 10x to preserve the waveform fidelity.
So designing a high performance digital circuit means that you are essentially designing high performance analog circuits that have a bandwidth extending up to at least 100 MHz, and in some cases all the way to 1 GHz. The traditional rules of PCB layout connectors, signal routing, ground planes, solder joints, PCB materials, and even PCB coatings break down at these high frequencies.