I love my REL. That said, when I wanted to get additional subs, I went with Rythmik. Why? REL is limited in the controls it offers to completely dial it in. For example, the phase is either 0 or 180; Rythmiks have a phase dial which can range in between 0 and 180. There are also a number of other features on a Rythmik —not least, their servo approach in the sub design — which you can read about, here: http://www.rythmikaudio.com/F12SE_features.html
Here’s a photo of the back of my two Rythmiks. I’m not saying that other subs don’t have this, only that this is why I chose Rythmik over additional REL’s.
I will continue to enjoy my REL in conjunction with the Rythmiks. But I have learned how hard it is to make bass work in my room, and the lack of flexiblity on the REL sets them below my Rythmiks. Phase has been my key to making everything work perfectly in 0-300hz.