The regular potentiometer has well-known shortcomings; distortion at the wiper/resistive-film interface and the film/contact interface.
Also, there are serious capacitive loading problems with the tone of the music changing as the volume changes.
The active potentiometer solution is the most popular, as it eliminates the capacitive loading, but not the contact distortion.
Unfortunately, the active circuitry adds distortion and noise.
Passive switched resistor array
The switched resistor array eliminates the contact distortion problem, but not the capacitance problem.
Also, there are not enough steps and remote control is difficult. Further, the switch contacts wear out after only a few thousand operations.
Active switched resistor array
The active switched resistor solution solves the capacitance problem but adds noise and distortion.
As well there are not enough steps and the switches wear out prematurely.
Opto volume controls seem a good solution, but there are distortion issues with the non-linearity of the light-sensitive phototransistor and there are the usually associated amplifier problems of hum, noise and distortion.
At first sight, the digital volume control appears to be the ideal solution, but even the best digital volume control must have an amplifier to drive subsequent equipment and the output noise does not reduce as the volume is reduced.
Also, as the volume is reduced, the bit-depth of the music is reduced and this is audible. It may be equal to, but cannot outperform the best transformer volume control.
The best solution for fidelity is to set the digital volume at maximum and then attenuate with the best analogue volume control possible.
Transformer volume controls are best, as they have near-zero noise, have virtually no distortion and the impedance issue with capacitance loading is eliminated.
There are two transformer designs. The regular two winding transformer volume control (TVC) and the single winding autotransformer volume control (ATV). The advantage of the TVC is true balanced operation and galvanic isolation (no direct connection between input and output). This is important in the recording studio environment but is irrelevant in the domestic Hi-Fi situation.
The AVC has the advantage of a smaller core size compared with an equivalent TVC, which means it has lower leakage inductance. This gives it a flatter frequency response and a far better transient performance.
Switched TVC or AVC
Most units do not have enough volume steps, they are rarely remote-controlled and the switches have limited life. The regular copper wired designs are a bit dull sounding and the silver wired ones are typically bright and edgy.
Reed Relay switched AVC
Reed relays are by far the best switches for low level audio signals. They have very low resistance, the contacts wipe when making and there is no contact corrosion as the contacts are in a vacuum. Further, they have a far longer life, at 30,000,000+ operations, compared with a rotary switch, at 3000 operations.