First, don't use the tape out. There are several reasons not to even consider it.

Now, on to the other options. Connect the Super Cube to the 950's sub output and set the subwoofer channel to "on" - this will send LFE channel data (the ".1" in "5.1") to the sub.

I'm going to make one other recommendation, although you may not like it: set the side and back surrounds to "small." If you leave the two Polk subs back there, use either a 40Hz or 60Hz crossover point. I suggest this because the rated low frequency extension for those two is only 35Hz. When you use the 7 Stereo mode, you may at times steer audio below that 35Hz point into the surround channels. It's less likely to happen with 5.1 mixes, but even then it is possible.