You’re on the right path to isolating the problem. What else are you plugging into the conditioner? Try eliminating a component one at a time while the sub is plugged in to see if the noise disappears. Are there any high current plug options on the conditioner for you to plug in the sub? Try one of those.
if the noise dissipates when plugged into the wall, then it is definitely one leg of a triangle of connected devices. This part of the triangle is the conditioner and sources. What outlet is the conditioner plugged into? Is it shared with other devices? Is it a dedicated 20 amp circuit? Try plugging the conditioner to another outlet. You can try just a simple multi outlet extension cord to see if the noise comes back with just a simple multi outlet extension cord. If so, then perhaps your sub is sensitive to some of the electrical characteristic of a multi outlet conditioner or extension cord. What brand and model is your sub? How old and how many watts? If the conditioner is not rated to support 1500 or more watts, than the sub amp and conditioner will get hot and may cause some electrical anomalies.