Subwoofer insight.

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.

ricmci, if you wonder if more subs really is better, check this out. https://systems.audiogon.com/systems/8367My first sub was the Talon Roc in the corner. A fine sub. But only one. One does not work very well. (Sorry, guys, but it doesn't. Can't.) Once I learned about the distributed bass array and decided to build my own the original play was to get the DBA up and then sell the Roc. Because who needs 5 subs?

So I built the 4 and removed the Roc and the DBA was awesome and was just about ready to sell the Roc when for kicks decided to try and see what it would be like with 5. Well it didn't add bass, its not about more, its about better, and 5 is a lot better. So the Roc stays. 

That's what you want. Don't worry too much about which ones. Four of just about anything will be better than one of anything. Four really good ones will be even better of course but mostly its about numbers not quality. Don't worry about placement. Placement with one is a huge headache, and nothing works anyway. Placement with two or three gets easier, because with more subs there are more modes and its easier to have them smooth out. Placement with four or five is trivially easy. Pretty much anywhere you plop them down you will hear awesome bass. 

I worry that by awesome you think I mean better than usual. No. I mean awesome. The exact word that was used three times by the last guy to come and listen. Deep, articulate, powerful, smooth, and 3D in an enveloping immersive way you have to hear to believe.

So get your two. For now. Consider it a good start.