A pure passive design is an implementation of various types of filtering. The best of this type will have individual filtering to each and every outlet and cover a very broad band of frequencies. The very best of this category also will pay particular attention to having very low DC resistance for full peak power delivery. This is extremely important so system dynamics are not lost. This potential dynamics loss has to do with the fact that most power amps can draw far higher currents at peaks than the average value. DC resistance anywhere in the power circuit will compress this peak causing distortion and power loss.
As a class, active regulation can be a partial correction/replacement or full regeneration of the power. Active regulation will also include some of the filtering of the passive units, thus providing some degree of separate input/output filtering. There are various types of power regulation products in the market place and some work very well for low power, front end equipment. Active regulation will often limit dynamics and current delivery when used with powerful amps. Active regulation can introduce other problems into the regenerated/regulated power, including reduced peak power delivery and broad band noise. The best active regulation conditioner will offer very low power distortion and often includes active power factor correction.
Power line input/output isolation is always implemented with a transformer in various configurations. These transformers can be anything from a simple standard EI core all the way to a more complex balanced power toroidal type. This isolation can also be implemented with a magnetic amplifier type circuit connected as a ferro-resonate regulator/isolator. All of these units need to be very big and heavy to allow for full system dynamics and power delivery. Ferro-resonate units can be and most are mechanically very noisy requiring installation in a remote location. Larger units may require dedicated power lines to be run to power them. As a class, isolation transformers do not provide broad band noise filtering but can be effective in lowering residual power line frequency hum problems.
Power factor correction is either implemented with active or passive circuits and usually is incorporated into one of the other classes of conditioning products. It is not offered as a stand alone device. Only activepower factor correction is practical for restoring the power waveform peaks reduced by heavy peak draw from large power amps. This correction insures full power delivery to all equipment connected to the same source. Power factor correction can rightly be thought of as power line distortion reduction and is required in some countries for certain classes of electronic equipment. It typically is actively implemented in the power supplies of these components. Passive PFC is usually done by power providers as large banks of capacitors seen on power poles or in sub stations. Typical passive correction requires very large capacitors and is impractical for home use.
With a goal of improved sound in the utilization of a power conditioner it is first necessary to do no harm. The question needs to be answered, what factors are the most important for good sonics. Research will show that voltage regulation is not as important as noise filtration and every effort must be made to minimize current restrictions that limit dynamics.
There are numerous pros and cons among the various designs of power conditioning products. Very important factors influencing the primary goal of "first do no harm", are the utilization of high quality parts in the design along with proper mechanical execution.
Ultimately, our ears tell us more than we can explain or measure scientifically. I encourage you to use yours to listen for what sounds the best to you.