You need to do careful a/b testing to make sure it's not snake oil
And if it needs to be that careful then one is likely to be listening to the system, not the music!
I went hifi shopping yesterday, looking at a new dac/headphone amp/headphone setup.
Took my existing setup with me (line 2 in signature below).
Tried a few headphone amps, and didn't find anything that sounded "better".
Most sounded no different, in the relatively short time I had with each.
Found one dac/amp combo that sounded worse, and (un?) fortunately it was the one I thought I'd be most interested in trying, from reading/watching lots of reviews.
Result - itch scratched, money saved
Then moved on to trying headphones, and the difference between them was instantly obvious, but it seems most other people must have big heads as even on the smallest headband setting some of them were just falling down past my ears!
I did end up buying a pair, but once again, not the ones I went there to try from reading reviews as they just sounded lifeless to me.
Glad I didn't buy them from stereonet without trying them!