Hello @kirkwallace! I run my two REL Carbon Specials with their Longbow Wireless. I now they have changed wireless systems over the recent years. Mine allows high or low level inputs to the Longbow. I prefer the high level, and the system works flawlessly. At first, I was fundamentally opposed to anything wireless, simply because I felt wired must be better. But putting the subs in traditional places, just inside my speakers, created serious bass nodes and booming behind my head where I sit, as there is a central wall for HVAC behind my couch. So I used the wireless to allow me to slide the subs around. Voila! placing them each well off to the side, actually on the side walls, worked like magic. Now my subs are basically impossible to locate, and blend perfectly without booming. Sub placement is finicky, and wireless allows locations that you would never even consider, let alone try with wires. I would even recommend the wireless just to figure out ideal locations, even if you want to later run wire; but no need as the system is perfect.
AS for the class D amps, I did formerly own a pair of Mola Mola Makua Hypex amps, and their hook up to subs was different. I believe I had to float the ground, but was able to use high level from one amp to one sub. As far as the stereo inputs to one sub, I have always been in the camp of just use one channel. I doubt you would hear the L / R difference, but one sub never images or disappears like two.
Best of luck! I do love the speed and sound of REL.