Use Sennheiser HD700 headphones with Expert 200

[LEPPAL]

Hi, hoping someone can assist me setting up my Expert 200 to plug in my Sennheiser HD700 headphones - I have configured Digital 1 and 2 to Pre Out and activated Pre Out (On/Off) on my remote, now I'm faced with a problem connecting them as the male plug (6.3mm) for the headphones need to pslit into two male RCA plugs and I can't find any to purchase anywhere. Wondering if I'm taking the wrong approach and should purchased a Headphone Amp and be done with it? - any advise is much appreciated.