What is the difference between Sorbothane and EVA foam (such us the one sold by MKUltra here https://mkultra.click) when used between the PCB and the case (under the PCB)?
I currenlty use Sorbothane in my Klipple+, but recently Mekanisk started selling EVA foam (in here Mekanisk Dampening Foam – Mekanisk Keyboards), and I’m curious about the difference between both materials.
Sorbothane is an incredibly dense foam that’s more expensive, while EVA foam is a less dense one that is much cheaper. Someone else correct me if I’m wrong, but sorbothane helps with sound damping along with increasing the density of the case, while EVA foam just dampens the reverberations by giving sound less space.
IME sorbothane is the superior product, but not by much for our applications. Take the law of diminishing returns with pricing into consideration & IMHO EVA foam is a no brainer for both applications. In between the PCB & plate & in the bottom of the case. As @Lesbian mentioned sorbothane will add a bit more weight to your case, but only buy a negligible amount compared EVA foam.
Sorbothane is a great material, but using it in our hobby for sound dampening is akin to firing up a wood burning stove to make a Hot Pocket LOL!
Sorbothane is specifically made as a vibration dampener, as in “keeping things from being shook when you smack it” dampener, so using it for a keyboard is a bit of overkill to say the least. But it’s delicious overkill.
Yes, Sorbothane is much heavier than foam.
I don’t mind the extra cost but can’t say Sorbothane is a joy to use. Cheaper easier to work with alternatives like EVA and Silverstone are good enough IMO.
I used my sobothane sheet with the plastic wrap still on it because there’s no way I’m going to put anything that sticky in a nice board. 
I think the very best may be a combo: 1mm Sorbothane hugging the case bottom to deaden vibration coming through the case and 2~3mm open cell foam on top. EVA is I think closed cell which I read is more appropriate for soundproofing. I just want to reduce noise and reverb.