Double Flush: any other mods?

[jwyse]

I'm building two Double Flushes, for myself and a buddy.  I want mine mostly stock, to start with, but my buddy wants to have some components socketed so he can play around with the values.  He requested socketing the components that shape the tone.

Which components would you recommend socketing and playing around with, for tone or other tweaking?

Are there any other mods (any other components that should be socketed) that I should look into?

Thanks!