@ronkent, Hi OP, I know this is going to ruffle some feathers but hear me out. The only reason I'm posting this is to try and explain and expose a lot of misunderstanding about acoustics and subs and dialling them in.
Big fat subs are generally seen to be used to develop prodigious amounts of bass and the RELs you have and the ones you're considering getting will certainly do that, but, they need to be integrated into the system with some understanding, which is what I will attempt to do.
Your room, as will all rooms, have unavoidable modes. I've seen some say : well it depends on the room. No Sir, it does not. Physics does not care one dot what anyone says. When bass frequencies are produced in a room the long wavelengths will combine constructively and destructively causing peaks and nulls respectively. These will be unique to your room, dependant upon the size, shape, reverb, absorption and, importantly, the position of the bass sources, these being your speakers and subs.
You do not want peaks and nulls. You want a smooth response, as smooth as possible so you are effectively hearing the sound without the damaging effects of your terrible room acoustics. It will be obvious that a peak 20dB higher than the mean will take much longer to decay leading to complaints of boomy bass, one-note bass or slow bass. Well, let me tell you: there is no such thing. Understand too that the nulls are just that, nulls which are caused by cancellation. What is cancellation? It's nothing, yes there is valuable bass information, the essence of your music is missing, it's been cancelled.
Now if you tame the peaks and fill in the nulls you will hear your system like never before and you will be stunned. With the response smoothed your nasty room acoustics will no longer stamp their signature on the sound and what you will be hearing is the acoustics of the venue. This is the audiophile's holy grail, is it not?
Hang in there, I'm not finished 
Achieving this peak and null free response requires a counter bass source to oppose the problem areas and this is best done by measurement which I'll not get into here other than to say it will remove any guesswork. It can not be accurately done by ear but if you're not up for the measurement stuff get someone to do it for you.
I have 2 main requirements for subs: They should be sealed boxes and above all else have variable phase. Your RELs unfortunately do not fit the description.
1/ Sealed boxes provide the cleanest bass. PRs or ports are another bass source that resonate at one specific chosen frequency and by definition can not be tuned save for adding or removing mass or shortening/lengthening the ports.
2/ Subs that only have 0-180 phase flip make them very much harder to dial in. While watching the response curve of your room you would need to pick up the sub and move it about and about and... I have no idea where it should go and neither does anyone else.
The good news, at last I hear you cry, is that with variable phase you can place the subs where you would like them to be, where you will not fall over them. HINT: Place one in a/any corner and one in the middle of a/any wall.
Now the fun part. If you are watching the graph on a laptop you will see the response start to flatten as you play with XO freq. XO slope, Phase, volume, room gain and PEQ. Modern subs like SVS do all this and from the convenience of your listening position.