RB DJ Track Preview using Headphones on DJM S9?!?!

I’ve read countless threads on the track preview function within RB DJ....clicking on waveform and previewing the track without having to actually load the track into deck 1/2 to preview. Here’s my issue:

In the preference pane within RB 5.2.2 I only have 3 channels that I can select to preview tracks on my DJM-S9, channel 1, channel 2 and the sampler channel. I’ve seen threads where others resolved their issue by selecting an unused channel such as channels 1/3 or 3/4, but how do you make this work on a 2 channel mixer that do not have unused channels such as you would on a 4 channel mixer?

when I select channel 1 for previewing all is good when the master track is playing on channel 2, however, when prepping to select a track on for the next song when the master track is playing on channel 1 the previewing of the track will play alongside the master track out through the speakers and of course headphones. The only workarounds is to go into preference each and every time I preview a track and select the channel that’s not being used at that time to preview a track. Of course that’s not intuitive so I would like to know if I’m missing something. 

Now when I select the sampler channel the preview plays out as Master since that’s how my sampler panel setting are set up...”A” 

Thanks