I have "Bobber Solo Saddle" with HD spring kit.
The springs seem to be rock solid. No movement at all
Did anyone replace the original scissor springs to softer ones?
Where to buy?

 

If you want a comfortable ride with that saddle, you can change all the doings you want, it will not be. More looks than proper use.
I have the solo seat and that's just fine, the added pollster will make the difference.
I would suggest if you can mount a solo seat for a trial and feel what it does.
Maybe here you can't find alternative springs:
WWAG