There is no fundamental difference in sound quality between "balanced" and "single-ended" circuits. The major upside to balanced circuits is that they are less susceptible to correlated noise picked up by interconnects, ground loops and power supplies since the balanced circuit floats inside its own insulated world.
In a single-ended system one side of the circuit is also tied to, and exposed to, the external shield. Thus, if currents flow through the external shields from one component to another the different components will end up at different voltages causing hum and other less clear cut effects. To create a quiet single-ended system usually requires choosing a single grounding point for one component and "floating" the other components. This is particularly important if a phonostage with very low-level signals is present. This floating can possibly lead to safety issues under some conditions where one of the floating components is disconnected from the rest system, and thus no longer has a true ground, but it is still under power.
The downside of a balanced system is twofold: As mentioned above, a true balanced circuit has roughly twice as many components and hence is more expensive. And, while usually not a practical issue, it has in principle more uncorrelated noise (by sqrt(2)).
In a single-ended system one side of the circuit is also tied to, and exposed to, the external shield. Thus, if currents flow through the external shields from one component to another the different components will end up at different voltages causing hum and other less clear cut effects. To create a quiet single-ended system usually requires choosing a single grounding point for one component and "floating" the other components. This is particularly important if a phonostage with very low-level signals is present. This floating can possibly lead to safety issues under some conditions where one of the floating components is disconnected from the rest system, and thus no longer has a true ground, but it is still under power.
The downside of a balanced system is twofold: As mentioned above, a true balanced circuit has roughly twice as many components and hence is more expensive. And, while usually not a practical issue, it has in principle more uncorrelated noise (by sqrt(2)).