I love the idea of this - a great practical solution for multiple subs in a small room, but one wonders how well it works. And REL continues to beat the drum of limited phase control. At the price they’re asking I find that puts them in a less desirable position. But the form factor is very intriguing!
They look great.
My subwoofer friends and I have this debate: I think my speakers give me all the bass I want, they tell me I need subs anyway.
It often isn’t about more bass, but more even room response in the bass region. But you know that. 
And oh, if that’s the argument from your subwoofer friends, then IMO they’re likely not taking full advantage of what subs can do!
Review
I’ve been told I’m the worst person on the planet for having only 1 sub and not 2 subs.
Some might say one could benefit from 2 good subwoofers in one system over one subwoofer in two different systems. 
If someone said you are the worst person on the planet because of your lack of subwoofers there must be a deeper reason than that. Pun intended 
Dear Paul
You’re not the worst person. I can think of one, maybe two who are worse.
I hope that helps lift your spirits.
Your friend
Ron
That sounds deep
Thanks Ron. I feel better knowing that 1, possibly 2 people on the planet are worse than me, lol.