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. 
 

Subwoofer/Low Frequency

Optimization see here

Mike

How do you have the REL connected? Maybe change the way you have it connected. Also, this appears to be a used sub, so make sure that it is actually working properly. 

Thanks for the responses. @soix I have not tried that crawl method but would be hard to do in my room. According to REL, the slope is rolloff a little to high. Rhythmik was one my option for a sub for some time.

@gkelly I have used high and low level connection and the gain is still the same level.

Hello @highend64 ,  something doesn’t sound right to me. I am a REL fan with a pair IN two systems. But even with one, it will easily play too loud. First thing, are you sure you have the phase correct?  I’m sure you know, but that is the first thing to adjust. You should easily be able to hear the bass go away and come in strong when the phase adjustment is correct.  If your REL  sub doesn’t have the ability to play too loud, then something must be wrong.