Audio gear of different types from different manufacturers is highly variable in the quality and filtration of the power supplies. Some are over-built like military equipment, some are so cheesy you wonder how it works at all, others after seeing how they're built you might never plug in again due to risk of fire or explosion
What makes this even more complex is that even if the audio gear isn't performing to spec, it doesn't necessarily mean the AC line noise is the cause. It seems to me one would have to measure both AC line noise and equipment output (record the signal and compute an FFT) together at the same time, on different days at different times. A power conditioner makes sense as a solution only if the gear is out of spec and it happens at the same times the power shows high levels of noise. Otherwise, a power conditioner is just an expensive solution looking for a problem.