MIT Z Duplex Super
I got an email from Jerry Raskin's Needle Doctor about this and I'm surprised not to have come across it before.
Anyway lets just say its the best power tweak I have found and I have both Nordost Qv2 and Alan Mayer parallel filters.
Better video, better audio, better everything. It's super.
It's not a series filter which limits current, but a parallel filter, which means you can wire it into a spare socket and avoid using the duplex outlets themselves which may be cheap compared to a good outlet.
Or you could use it as your main wall outlet, which I am now.