These little devices are touted as a form of noise reduction/cancellation. They are attached to the AC hot and common terminals and mounted inside components. I can't evaluate the technical claims for these, but the sound is what counts for me anyway.
I've read about these and their benefits in DACs and Class D amps in particular, and finally decided to give one a try, putting it in my dual mono Hypex NC-400.
All the glowing reports are true.
Clearer, cleaner, with notably blacker space between instruments, allowing very subtle spatial information to emerge more clearly out of the smog with an opened up soundstage. It is not a subtle improvement, and very enjoyable.
I bought a second one for my Cullen modded PS Audio GCC-250 integrated, already a very good Icepower amp, and the result was much the same. In particular, the bass, which was a bit soft and unclear before, now has great definition and slam. The top end definition, a bit of a weak spot in Icepower amps, is also sharper, clearer and more open and spacious. I had been thinking about trading this amp, which was very neutral and powerful but not up to my N-core in transparency, but now that is off the table. The enhanced clarity is very welcome; it would have cost a lot more to find a replacement amp that would be this much an improvement over what I had before the Pulse Gen.
The basic character of the amps is still there; the NC-400 is still the more transparent and dynamic of the 2, but both benefitted in similar ways and to similar degrees.
No downside at all that I can hear, but the top end is less hashy, more natural and relaxed, so the overall sound is less bright with both amps. This may not always be a good thing in a laid back system.
Other opinions in Positive Feedback and Affordable Audio.
These are a very effective tweak for a not unreasonable $400. A good option to try in already fine systems, IMO. I'll try one in a DAC next.