Subwoofer/Low Frequency

Optimization see here

Mike

One possibility is that where you have the sub positioned is in a bass node in your room that’s sucking out the bass.  The first thing to do is trying different positioning to see if that helps, and it’s free.  Here’s the tried and true “crawl method” to identify the best spots in your room for sub placement, or generally a safe placement is placing the sub 20% the length of the room from the back wall and 25% the width of the room from the side wall if you can’t do the crawl method.

https://www.audioholics.com/home-theater-connection/crawling-for-bass-subwoofer-placement

If placement isn’t the problem, what you’re running into is that the REL is really a woofer and not a true subwoofer, which is likely why you’re missing bass depth/impact.  Getting another R218 may help a bit, but it still may not give you what you’re looking for and is expensive so if I’m you at this point I cut bait and get a Rhythmik F12G that’s about the same price, is very quick/musical, and will get down to 14Hz (-2dB) versus 25Hz (-6dB) for the REL. 

https://www.rythmikaudio.com/F12G.html

The other obvious option would be to get a bigger REL, but that will likely be quite a bit more expensive.  Hope this helps, and best of luck. 
 

A single sub needs careful placement...and takes time to properly balance the gain and crossover settings...2 will both sound better and be easier to place...I started with a single REL S/5 SHO and it sounded great, but a second one added a year later was a nice improvement...