Here's an alternative option if you can use two subs and your main speakers allow both spade and banana connections. In my winter system, I use a pair of Gallo TR1D subs. The subs have a speaker level input. I run my main speakers full range with the speaker cables terminated with spades which leaves the banana sockets at the speaker unused. Therefore, I can send the signal to the subs with a short banana terminated cable of the same type as the main speaker cable to each sub directly from the main speaker. This way I don't place the cross-over of the sub inline with the main speaker.
The cross-over adjustments on the subs still function as regards to the subwoofer outputs but do not limit the signal to the main speakers.
My main speakers are first order and the Gallo subs are sealed. The subs are placed close to the main speakers and integration is fairly simple.
I also dislike REL subs FWIW.