I have a cute DMM that measures frequency. I plug it into a wall socket and watch the line frequency float between 59.9 and 60.1 Hz.

I have experience with larger steam and gas driven AC generators and they produce a very clean output power. Spurious noise picked up in the line should be rare and anyway it should be filtered by the equipment. Perhaps you could connect an ocilliscope which would let you display the entire frequency sprectrum in your house plugs?

I do not think I have seen anything in a power conditioner that justifies $600 and up prices. But then I have never seen an interconnect that was worth $300 and up a pair, but people still buy them.