Off Camera Flash for NikonD800
This pretty much said it all except with the caveat that you said "inside" a softbox. If you're actually putting it inside a softbox such as a Westcott Apollo -- as opposed to at the back end of a softbox (where your SB600 is exposed) you can't use CLS since there would be no consistent way for the sensor on the SB600 to "see" the trigger signal.
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