Research for subwoofer

Thanks for explaining, but which is which  Is REL the Sub Base?

Keep in mind that two subs are the best way to go. Getting one is great and will make your system better. Just that two subs takes it to a whole new sound level. I am a Rel Acoustic fanboy, so this is my recommendation https://rel.net/collections/serie-tx

this company has a reputation for being easy to integrate with speakers for a seamless setup. If I can use my experience they are correct. 

Before buying anything, you should hear Rhythmik. Push pull subwoofers are vastly superior to any other! So much cleaner and cheaper than Rel. 

Plus one on Rythmic recommendation. Roll off your bookshelves at 50 or 60;hz and get a Rythmic sealed sub or 2 sized for your room.

If at all possible you should go with two subs (almost all are powered BTW) as two are a huge improvement over just one for many reasons.  I’d highly recommend the SVS SB1000 Pro subs as you can get two new for only $1150, they go down to an honest 20Hz (-3dB) despite being only about 13 inches square, and they come with excellent integration software/app that’ll make it much easier to successfully dial them in properly, which is super important.  And they offer a 45-day completely risk-free trial period including shipping both ways so if for some reason they don’t work out you’re out nothing but some time, and they also offer excellent customer support if you need any help with setup, etc..  You should expect not only much better/more extended bass but also better and more solid overall imaging, dynamics, and an expanded 3D soundstage.  

https://www.svsound.com/products/sb-1000-pro-subwoofer

FYI, REL subs anywhere near this price level won’t go below the low 30Hz range so are more like additional woofers than true subwoofers, and IME although there aren’t a lot of musical instruments that go that low there’s important spatial information below 30Hz that is very meaningful and impactful especially in the areas of dynamics and an added sense of space and expanded soundstage.  Plus your speakers I believe already go down to the low 30s so there’s that.  And I agree with others the Rhytmiks are even better but will be considerably more expensive and do not come with integration software, which are not small considerations.  Just my $0.02 FWIW.  Hope this helps, and best of luck.