Recently we've gotten a couple of questions which essentially reduce to http://photo.stackexchange.com/questions/61011/how-many-ev-will-a-softbox-knock-down-off-your-flash — but that doesn't really have great answers. I know it's not a simple answer (like "add 2 EV and there you go"), but it'd be nice to have some discussion of flash GN and what _might_ be appropriate for example situations. Will a mid-range ~ GN 30 flash ever be useful with a softbox, or is more power basically required when using lighting modifiers?

I'm happy to add a bounty to the above question, but I could also see a new question asking something like "How powerful a flash do I need for use with a softbox in typical situations?"

Which do you think is better?