Subwoofer insight.

I find most any subwoofer/s is better than none. 
Its thrilling when you realize how much low frequency information you've been missing. Similarly, when you discover the further improvements as a result of the proper room placement and calibration of the two subs or more.
Your in the mob now.

Two is a good start. Much better than one. Or none. One thing you will learn, if you're smart enough to try, is the more the better. One sub is better than none and sometimes kinda sorta matches. Two is better still. With four you find all of a sudden not only is the bass smoother faster deeper and more articulate, but it also blends seamlessly in a way that makes all the speakers disappear along with the room as you are enveloped in the sound field. 

So yeah, go for it. Great place to start.

Darn good idea.

Your existing speakers may sound great, however they are bass limited, and inefficient as you know.

1. a stereo pair of self powered subs is a very good idea, don't over do it. two, located near the mains, so the low bass and the overtones from those notes are directional.

2. removing the need for your existing amp to handle the bass is going to allow it to do a better job with your inefficient mains.

3. easy independent volume control/balance with the mains is nice, if buying new, remote vol of sub would be very nice. 

Adding a Rel sub to a pair of Kef LS50s was one of my better decisions. The sub filled in the bottom end and increased the soundstage (depth, in particular). The little Kefs no longer sounded small. Would like to add another sub but room layout is very limiting.  

FWIW - I’m running a pair of SVS SB1000 subs alongside Totem Forests (2 ways and similar sensitivity as your speakers). The subs are receiving a speaker level signal from the amp. Both subs and Forests run "full range" via parallel runs of speaker cable. The subs are dialed back such that you only know they are in use due to the blue LED lights or if they are switched off. Definitely, bass is improved but what shocked me was the improvement to midrange clarity and "openness" running the subs brought. I can’t explain why this is so but this was further enhanced when I upgraded power cords running to the subs. Given the Forests by themselves go a little deeper than what you cite for your speakers and given the audible benefit (not just better bass) I’m hearing from the SB1000s, I’d encourage you to try a pair of subs. (I’m assuming the RELs you are considering are self-powered). Hope this is helpful.