Hey Matt,
I would be careful pairing a REL with your A5’s, Most if not all REL subs incorporate a passive radiator in their designs. The passive radiator dramatically increases the decay and sustain portion of a note or beat. (The four stages of every note or drum beat are attack, decay, sustain and release) Your A5’s being a sealed cabinet design will not do that, so the bass coming out of the REL will sound much fatter than what comes out of the Magico’s and they may not blend well together.
Concerning the high level input, that was created out of necessity. Most home audio at any level, low, mid or high is not designed for the incorporation of a subwoofer. Unless you have separates, or an integrated amp that has preamp outputs and inputs, it’s pretty difficult to send a secondary line level signal to a second amplifier. So if you are a speaker manufacturer and all you make are subwoofers and you want to increase your market share what do you do? Well you create a high level connection that takes that speaker level signal, convert it back to a line level signal that an amplifier and other circuits can handle, process said signal and re-amplify it. Not the best way to make a connection but if that is the only way you can make it, at least you have an option.
James633 is giving you excellent advice.
You also may want to check out the link below.
https://www.acousticsciences.com/wp-content/uploads/2024/11/Soundoctor-Barry-Ober-sub-integration.pdf