My thoughts are this. Deliver a clean sine wave. There must be regenerators that do this, but at cost. The kind of issues that can cause a toroid to buzz I'm told are caused at the zero crossing of the sine wave. Often described as a DC component in the AC. I already have one of those kinds of "filters", and it does actually work, and I don't believe it exists as something built into any surge protector/filter. There may be some amplifiers that have it built in, like Bryston, which a schematic I once had a peak at.
I think filters are only going to take things so far, since you are filtering other components from the AC, but the AC signal alone could be pretty distorted itself. I've read that the problem is getting worse as power company load types are changing from things like flourecent light bulbs and other things that were not as common place. This is just from my reading, I do quite a bit when trouble starts. There does seem to be many many issues tied to ground, but the power lines are not something I can mess with.
For me finding something that would fix my AC problems would be great, which might just be a good electrician. However I'm willing to explore a "plug in" solution. Cost for me is always a concern.
