They are all the same, so get the cheap one. When I heard about this last year it seemed kooky, but mahgister and roxy54 are credible and higher skilled audiophiles and so I started searching around. Because even at $10 I take my time and DYODD. Right away there was a review of a $400 product that showed the exact same circuit board inside it as all the ones on eBay and Amazon.
So don't even bother, the circuit boards are all the same. Unless you want to pay $50 to $390 for a box. Don't bother with returns, they are $10 and not worth the trouble. Especially since the only way you will be returning is if you are stone deaf! (On second thought, that could be a lot of the most prolific posters....hmmm....)
What these things do is clear and easy to hear. People so stuffed full of it they cannot do anything until they can talk themselves into thinking they understand what is going on, good luck! Can you tell me how fiat currency works? Your car? AC? Anyone? Beuller? But somehow the world is supposed to stop until we can explain how this thing makes music sound so dimensional, detailed, natural and real? Right. Dream on.
Starting with one I added a couple at a time until now at 8 with 2 more on the way. Each one improved the same amount as the last. So much for the "law" of diminishing marginal returns. Some say they need to be 5ft off the floor. No idea where that comes from. Six of mine are about 5 ft up, but two are right on the floor and when turned off and on (I have them on a remote switch) it is easy to hear they all make the same difference.
So just get em. Then when you come back and post how awesome they are be sure to thank mahgister and roxy54, and whoever else was willing to endure the slings and arrows everyone with the temerity to act like an actual audiophile is subjected to around here.
Oh and guess what? In spite of the above I have a pretty good idea how they work. And I may even tell if you ask me, like Riddick said, "real sweet like."