need to convert stereo rca out to mono LFE with an active crossover somehow

[CptnReflex]

I have a Mirage Omni S12 subwoofer that has this on the back:
 

 

my Focal shape 65 on the back look like this:

 

 

My USB DAC looks like this on the back:

 

The USB DAC plays audio on both the XLR and the RCA at the same time and I've tested that, but the crossover in my sub doesn't cancel everything out as well as the fact that only left or right channel can deliver bass at a time for it. My subwoofer goes down to 19Hz and is better than very high dollar studio subwoofers for sound quality but I don't have passive speakers to wire into the high level inputs.

 

Is there a crossover that can convert stereo RCA into mono out specifically to put into the LFE RCA input into my subwoofer? My only other options are: 

use a home theater receiver to convert the RCA into sub out which is a huge waste of power and space

 

Sell my subwoofer and get one with RCA stereo inputs

 

Get a second Mirage Omni s12 for stereo subwoofers

 

Some kind of crossover option I never heard of before that can convert stereo RCA out into mono hopefully also that can eliminate high frequencies

Return my SMSL M500 MKII and get some kind of rotel surround sound processor for 3 grand with 5.1 balanced outputs and simply use RCA unbalanced out for the subwoofer which really isn't realistic at all

 

Get a crappier studio subwoofer that sounds worse than my existing mirage subwoofer just to have balance inputs and outputs which I really don't want to do

 

sell my focals and get passive speakers as good as them and use high level input (really don't want to sell the 3rd best speakers I've ever heard in my life)

 

I tried asking reddit and they haven't helped me at all and I'm at a loss on how to find the proper part I need to make it work. I already have an infinity reference sub with the stereo inputs which sucks big time compared to this sub but it might be my only option