Ok, so I've never used Traktor with a 'proper' mixer (I come from a TP & VCI background, so upgraded to TSP, bought decks and use my controller as a mixer, sending the master out from the soundcard to the amp)
I occasionally use a third Traktor deck for samples, this is easy enough on the VCI as I use either deck C or D depending onwhat appropriate - I assume I'll need a 'third' channel of some description for this on the mixer if I want to continue doing the same - would the aux in on the Rane 56S suffice?