How about this, for your experiment.
If that sub has speaker level inputs and outputs, then you can use one of the front channel's speaker cable to the sub speaker level in, the sub will take care of the lows, and then the speaker level out to the speaker and it will handle the signals the sub bypassed.
This is your only option left with what you are trying to do without the amp input on the receiver for that channel.
Or, just thought of another out for you, is getting an external amp and use the sub out to the amp in and that amp to that speaker you are experimenting with.