Dual mono with REL Subwoofer

[PeppaPig]

I think there are 4 sockets on the pair that can do line-out. This can be left, right, or a mono mix of both. They are RCA sockets 5 and 6 reading left to right as shown on the configurator. 

But socket 6 is used for the digital link between the units, leaving only 2 sockets remaining. On my master I'm using socket 5 for a digital in, so my obvious choice is socket 5 on the companion, as shown in the picture.

The 3 wire connection is not used at all and the sub is set to "LFE" mode which disengages its crossover.

For a pair of subs you could use a splitter adaptor, but each sub would see a mono mix.

For stereo, if the socket 5 of both units is available, just set the mix of each socket to left and right correspondingly. If I really needed to free up my master socket 5, I could move that source to the AES input on the companion, which is currently unused.

So my master amp I set one post to Mix Mono, and companion I also set one post to Mix Mono, and set the cutoff at 40Hz. Then I connect each subwoofer to each master and companion using the LEF connection on the subwoofer?

Then what about the crossover on the subwoofer? Turn it to max? What about volume?