Yes glad to know I'm not the only one think this way!!!
I've also gone through my fair share of keycaps and ultimately always coming back to stock ABS keycaps.
Here are the keycaps I've tried:Filco stock ABS
Das stock ABS
Blackwidow coated back-lit ABS
Imsto thick PBT cherry profile
GMK thick ABS double shot
Rainbow thin PBT
Vortex double shot PBT
Vortex thick PBT OEM profile
Poker 2 ABS back-lit keycap
SP DSA PBT
SP SA thick ABS double shot
Leopold FC900R thick double shot ABS
In order of preference I'd say I like:Coated back-lit ABS > double shot thick ABS > stock ABS > thick PBT > thin PBTBasically coated back-lit ABS give the deepest nicest "thock" out of all the keycaps I tried. Reason being softer material whole wear out fast but gives a nicer deeper sound. PBT is very brittle so it gives very sharp sound.
As for PCB mount, I find PCB mount on smaller keyboard gives a nice echo similar to HHKB's case mount. However I tried a full size PCB mount it just doesn't have the same echo.
Another example of very good sounding PCB mount keyboard is KBT Race II.
The last keycap material I want to try is POM, but as of now I wish they make coated ABS blank keycaps (even though the reason they coat keycap is so they can engrave out the letters).
Oh the only exception that I know of is that I prefer PBT on Topre, my guess is that since Topre domes already dampen the keycap sound so it doesn't come off as sharp anyway.
I feel like we might be the minority here but for deeper sound definitely aim for softer and thicker keycaps.