Reddit mentions of RiteAV - RCA Splitter Female to Male

You could use something like this if your Genesis output cable only has the video output and one RCA connector for mono audio. That would allow for stereo output to a TV or capture box. If you want to use headphones, a second adapter that goes from two female RCA connectors to one female 1/8" headset jack would work.

What /u/Cheedweed said.

But just in case the rear/sub channels aren't actually switchable and it really is only fronts and a dedicated sub out, then you will not want to run the rear speakers from that sub output.

What you will want to do is purchase two sets of RCA y-splitters (1 female end to 2 male ends) like these (they are sold at my local Walmart for $4). And just split the front signal into the 4 channel amp. You will lose the ability to fade front/rear but the 4 channel amp (most likely) has adjustable gain for fronts and rear separately so you can setup up the fade manually that way.

Get something like this for the amp end of the RCA cables and you're golden http://www.amazon.com/RiteAV-RCA-Splitter-Female-Male/dp/B000V1PB3O