The OAW3 is designed specifically for the case of your 50ft cable run to the back sub.

If you had a sub in front that was easy to wire, I would suggest splitting the sub output with a Y cable. The front sub would be hard-wired from one half of the splitter, and the other half of the splitter would connect to the OAW3 transmitter. The OAW3 receiver would be connected to the back sub.