If this is a two channel system in a reasonably conventional room and your main speakers are making a soundstage at the listening position two subwoofers effectively placed should provide more than enough node free area at the listening position to begin with.
I'd suggest using both your existing subwoofers with the .7s in a mono configuration as a starting point. Concentrate on finding their optimum room location which reduces the most room nodes.
If the REL cabling prevents your from locating your subs at most locations in your room you might consider running them using their low level inputs and very long economical interconnects from Blue Jeans or Monoprice cable to aid in their room location.
You should be able to mitigate your concern of what you term as overlap with proper crossover and gain. Frankly, I don't think this will be an issue especially if the subs are irregularly located in the room and running in mono.
When I compared a REL Studio III it worked best sourced from a low level signal processed through a Velodyne DD Optimization program. I am not a fan of REL subwoofers.
I'd suggest using both your existing subwoofers with the .7s in a mono configuration as a starting point. Concentrate on finding their optimum room location which reduces the most room nodes.
If the REL cabling prevents your from locating your subs at most locations in your room you might consider running them using their low level inputs and very long economical interconnects from Blue Jeans or Monoprice cable to aid in their room location.
You should be able to mitigate your concern of what you term as overlap with proper crossover and gain. Frankly, I don't think this will be an issue especially if the subs are irregularly located in the room and running in mono.
When I compared a REL Studio III it worked best sourced from a low level signal processed through a Velodyne DD Optimization program. I am not a fan of REL subwoofers.