These dongles do work but it's very much on a case by case basis. I bought an original IFI iSilencer a few years back when I was ripping CDs and got a poor result from it when using it in series with my USB cable.
Fast forward to last week I found it when going through some old stuff and put it into one of the spare USB inputs on my Aurender and I was so surprised that I bought a new version too.
So just inserting them into the empty USB outputs does have a good effect decrapifying, again using them in series with the USB cable has poor results.