The single biggest improvement in the sound of my keeb didn't come from switches or foam or new keycaps...

...it came from a goddamn dish towel.

I've since upgraded to a proper mat, but holy hell -- I wish I would've realized what a difference proper underlayment makes earlier on in my journey. I spent a small fortune chasing a deeper, more pleasing sound profile, and nothing I did seemed to get me where I wanted to be. Turns out all I needed was a mat.

TL;DR: don't underestimate the power of a good, thick, vibration-dampening desk mat.