One Sub or Two?

One great sub is better than two so,so subs. But two great subs is better than one. Two good subs together won't go below 20hz any better than a single good sub that won't go below 20hz will. We need to get away from the phrase "tight bass" which actually translates to less deep bass in the real world.

The only reason for 2 subs is if you don't have space for 3 or 4, but you do. The poster who claims you absolutely don't need subs with these speakers is unfortunately ill informed and has obviously never heard a good multi sub set up. I suggest he read Geddes, Welti or Toole

The advice to look at Rythmik or GR Research is solid, a servo system that IMO outperforms most.  @hilde45 mentioned variable phase which is essential for truly dialing in the subs which will exclude most of the REL range. Another drawback of REL is their use of passive radiators which behave in a similar manner to a ported speaker which I also suggest you avoid. A port is another bass source but unfortunately it is frequency invariant and therefore can't be tuned. Home Theatre guys like them but my recommendation is to just stay away for music. I own a REL and a small SVS SB1000 and it handily outperforms the REL. Sorry REL owners, I gotta tell it

To optimise the rig you're advised to measure your installation using REW (free download) I use OmniMic V2 to measure but it is now discontinued and replaced with an improved version available from Parts Express for $599, I think.

The crawl method to locate the best spot for the subs is poor advice although popular. What you will be locating is the loudest bass and that will be from one of the room's many peaks but what you won't/can't hear is the deep null nearby. It's a null, a black hole for music so there is nothing to hear, however measurement will reveal all peaks and nulls and juggling the parameters will flatten this out to a smooth response. The variable phase function will allow more freedom in placement.

In a choice between one or two, I’d say two. I have two JL’s, but wish I had four🤷‍♂️

It may not make intuitive sense at all but two subs produce a lot better and a lot more control over bass. I know people always say that bass is not directional -i.e. you can't always tell where the bass is coming from but with only one sub it is likely you can detect the source of the bass you are hearing. The goal is truly making it difficult if not impossible to determine where the bass is coming from -i.e. being able to blend it with the rest of your audio signal and 2 subs seems to make it easier to do that. It is likely you will spend some time determining exactly how to position both subs but my experience is alternate corners of the room can work very well and you can play with adjusting the exact direction for the subs from there. As to which subs to select, I would suggest self powered subs and my choice was 2 JL Audio Fathem F-113 subs.