I have not tried any of that, I might be getting a DC filter, I think, but mostly I don't bother since I have different amps for the high imp headphones. Local power does seem to vary on such things, I have only tried it three places in the same area of my home town, so I shouldn't generalize, I am under the pression that 230v users such as myself are more at risk for hum than 110v.
I added Audience Adept Response AR12 to my system couple of years ago...it made a big difference.
you can read up on it here: http://www.stereophile.com/powerlineaccessories/407aud/#UIVkyZcuG4ZTVIMc.97
I red some posts on a french audiophile forum that advised to do this check and in my case it helped a lot. Sound was less fuzzy, it gave more cohesion and aeryness.
It costed me 10 bucks or so for the price of this tool but i thought it was worth it. I'll look up this info tomorrow night and post the links and at best will try to translate it for the english only speaker.