The ifi iDefender 3.0 is the only USB "purification" (really it does something pretty simple - cuts the USB power out of the equation) product that has actually worked for me. Did wonders for my ground loop hum issues, but my PC's USB output is still cancer and will make certain DACs noisy.
My friend has a Skylake i7 machine that has such a noisy USB interface that it actually makes DAC's screech randomly.
He would tell me that every USB DAC he tried he ended up with random noises or popping in the music.
I think he started to use the optical output from his computer to avoid this problem, and he bought an Eitr too i think it was...
I have personally never had a computer myself with such USB issues, but hes not the first friend to have issues like this in terms of USB power.
Although at no time does that mean that the USB power filters will fix this, or that fidelity would be improved.
It just means that devices that isolate the DAC from the PC are able to FIX problems like this.