Well, it's not completely placebo after all. I did blutack mod around 5-6 years ago on my Shanling CDP. It did bring the different.
Then, I was introduced to VTS Marigo Dot. This brought a large enhancement. You can try yourself and buy some (I'm not a seller or related with specific brand). Put it on your opamp and DAC. Hear the different. If I could recall my brain correctly, mine was tested on ECHO AudioFire, Kimber Silver Streak interconnect, Kimber 8TS speaker cable, and fed into solid state integrated amp and Monitor Audio floorstander (I forgot the model/type).
After I knew VTS, I just realized the Blutack produced "soft/quite" background but also "compress the dynamic", while VTS didn't.
I also ever tried Shun Mook for tuning up my speaker.
Few months ago, I was introduced to Music Snap (I don't know where this came from? My friend just gave me some to try). A little bit different concept with VTS, but also produce "unbelievable" result (or "placebo" for some people who speaks before they try?).
Within over 15 years my journey on audio, many unbelievable things in this audio world. Some are explainable, some are not (yet explainable). I think just put respect to other who has different perception. Music is about our own enjoyment, right?
PS: VTS and Music Snap most likely explainable with Micro Decharge d'Interface and Pierre Johannet keywords. Try to look around at Google before saying it as "placebo".
PS2: Blutack mod is "poor's man style". It brings improvement, as long as you put it right, but for better result, you should go with VTS or Music Snap or perhaps else such as Shun Mook.
My $0.02.
Thanks.