I’ve been reading about this question, too, for years and I share your curiousity.
The wisest response I’ve gotten when I’ve asked the question is: "How bad is your power?" In other words, is there an actual problem you would be curing or are you seeking a solution for a problem which doesn’t exist?
[This is a great response because it helps explain why audiophile's comments about power conditioners/regenerators/etc. are all over the map. Whatever else differs between people's perceptions, it is surely the case that local power conditions -- in a town or county or even particular home -- differ massively.]
My response to this question has always been, "How does one find out how bad their power is? With what measurement tools or local power company information can one determine how noisy their power line is?"
The answer to that question has typically been, "It usually takes professional equipment to determine this."
OP, if you’re the sort of person who only wants to spend money and time on a real and not an imaginary problem, I’d be curious to learn how you solve your own question.