This is sort of my ocd going nuts, but I do think there's /some/ science to it.
what I did is got two different types of o-rings (thin silicon, and 50 durometer rubber) and placed silicon on some keys, and the rubber o-rings under some others. (note: thick pbt is the material i'm using for the caps on my 37keys)
the theory is that some of my fingers have more control than others, so on those where I have less control I am using my rubber o-rings to dampen the bottom-out sound. with the other fingers to which i have greater control, I am using thin o-rings since I have no problem avoiding bottoming-out on them. I've also noticed this adds variability to the sound and feel, which is something else I like.
==============================================================here's the layout i used
legend:
pale yellow = single thin siliconlight orange = single rubber o-ringorange = double rubber o-ringred = triple rubber o-ring==============================================================the reason I used triple o-rings on the spacebar is to get rid of the ping I was experiencing. at first i thought using the thick PBT caps i bought would get rid of it. turns out that didn't help, and what was causing the ping were the stabs vibrating between the backplate and the PCB. i just went back to the original ABS spacebar, inverted it, and gave it triple rubber o-rings. ping gone.
here's a typing sound demonstration of me typing on it:
https://db.tt/6HLYF46d(note: the occassional snap is the key springing back, since i sometimes roll my finger off the edge of the key. bad habit i guess)