The REL is covering all of the bases for various connection types. You need only use the LFE input port to connect a cable to one of the Onkyo’s sub pre outs. I’d set the volume knob on the sub anywhere from 30% to 50%, though it might come in hot by 40, 45%. Set the crossover knob to its maximum at 120hz. Now, this won’t mean anything if you set the Onkyo’s LPF to 80hz. But, you will undermine the Onkyo’s LPF setting if you set the knob to anything lower than the setting of the LPF so don’t worry about setting it to maximum. Interesting that the max setting is 120hz, isn’t it?
Now, I do see that REL recommends a Large setting for the Front speakers when using the High level inputs for stereo as well as using the LFE input simultaneously. Onkyo’s designation for Large is Full Band. Just remember that setting the Front speakers to Full Band means having to turn on Double Bass in the Onkyo as well or it will not send a signal to the sub when playing back a two channel signal. This is because it thinks that if you set the Fronts to Full Band, they do not need the extra help. The name is silly. I might have called it FBAS(Full Band and Sub).
Another thing to remember when playing two channel signals is that you will not get a signal to the sub if you select the DIRECT mode for playback in the Onkyo. It matters not if Double Bass is on. DIRECT mode is just that, an unmolested signal with no processing, bass management or any available adjustments. So, while it will send a signal to the sub while playing back a 5.1 track, because there is a designated LFE, it will not do so playing back a 2.0 track. The Front speakers will receive a full, unaltered signal in DIRECT mode regardless of any other settings while playing back a 2.0 track.