I recently purchased a couple of Blue Circle SillyCone filters. They are a parallel filters which are plugged into any free outlet but their effectiveness is increased the closer they are to the equipment outlet. These filters are built into a silicone filled ABS pipe, they aren't much to look at, but don't let that dissuade you.
I purchased them for my monoblocks as I have always plugged the amps directly into the wall. Since the SillyCone filters are parallel filters, they do not restrict current and dynamics.
What immediately grabbed my attention was the level of refinement these filters brought to my system. Leading transients are less edgy and more natural. Backgrounds are blacker which allows inner detail and texture to flow through. Images are portrayed with greater density and space.
Previously, I had PS Audio Harvesters and at certain times of the day, their LEDs would start flashing so rapidly, that I could hear them buzzing. Even when I added more Harvesters, it didn't make a significant difference to the amount of power line noise as they would all be buzzing away. When I plugged in a single SillyCone 6x filter, the blinking on the Harvesters dropped significantly. Adding a second 6x filter stopped their blinking all together.
I eventually ordered two 18x filters for my amps. I'm thinking of getting more of these pipes, they even make a nice contribution when I plug them into my BPT conditioner.
http://www.bluecircle.com/page26.html