Unable to hear headphones in SX3 (Windows 7 on Mac)

Hi

I recently posted a question about installing Cubase SX3 on an iMac running Yosemite - I’ve managed to solve the issue by partitioning the Mac’s HD and running Windows 7 on one of them.

Everything is working wonderfully - except that I am unable to hear anything on my headphones when plugged into the iMac’s headphone output. I can hear everything else perfectly on the headphones, though (YouTube, iTunes, the Windows noises etc.), and there is an output through the iMac’s speakers.

I am pretty certain this is related to the VST Connections / Device Setup. I managed to open an old idea in SX3 (which uses Kontakt 4 as a sampler and a few other VSTs - it comprises only MIDI tracks) and upon noticing that there was no output in the headphones, ensured the headphone levels were up and added a Stereo Out Bus in the Outputs. The headphones then suddenly started playing the Cubase file so I put it down to a lucky fluke.

I then closed SX3 in order to install some more VSTs and upon opening it, was told something about a change to the audio set-up (I forget the exact wording) and was prompted to run through some sort of configuration test. Every since then, I have not been able to get any audio from my headphones.

Does anyone have any idea as to what I need to try?? Looking forward to your responses!!