Regarding high-level connections, another option is to use a Jensen Transformers Iso-Max SP-2SX 2-channel speaker level to (balanced) line level converter.
I use one with a DSpeaker 2 channel room correction box and dual REL Storm IIIs running in stereo.
To connect it, I made custom cables with spades from each speaker, to the SP-2SX speakon input connectors. Then the SP-2SX balanced line level outputs are connected to the balanced inputs of my DSpeaker Anti-Mode 2.0 using standard balanced XLR cables. And then a second pair of balanced XLR cables are connected from the DSpeaker balanced outputs to the "BAL LO" inputs on each of the Storm IIIs. So the extra processing is only applied to the less critical subwoofer signal. Works for me.
Alternatively, the SP-2SX also has a terminal strip that accepts bare, tinned or pin style connections for all the inputs and outputs.
FWIW, excerpts from published specs/claims for the SP-2SX...
• Delivers exceptionally low distortion down to 20 Hz
• Ruler flat frequency response from 5 Hz to 20 kHz
• Once connected, the SP-2SX is able to withstand signal levels to +19 dBu at
20 Hz without introducing distortion, phase shift or artifact of any kind.
Regarding single vs dual subwoofers, I happen to like the even greater expansion of sound stage that I get with dual subwoofers in stereo, but of course you could alternatively run both SP-2SX output channels to a single subwoofer.