@wolf_garcia --
A lot of silliness get passed around with topics like this one, and all bloviating aside, subs aren't that difficult to set up, more than one helps tame standing waves but one sounds much better than none, and don't be afraid to try 'em. I have a few older RELs (no high pass nonsense needed) bought used and they're great.
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.. I have decades of experience in studios and live concerts as a musician and live sound mixer, and am baffled by how baffled some are with this stuff, and the noted complexity some need to yammer about doesn't help anybody.
On the second paragraph: you may have given yourself the answer without realizing it; not everybody has "decades of experience in studios and live concerts as a musician and live sound mixer," which otherwise would've likely given them a head start implementing subs in the home setup.
Re: first paragraph: it's not really about "complexity," but rather what you choose to go with and how you intend to do it. Being very experienced yourself with sound mixing both in studios and at a live concert milieu while having a bunch of REL's (whether they're used or new is besides the point, btw) to play around with, isn't a bad outset by any means. REL's would seem easier to implement in an existing setup compared to other sub brands, and combined with your experience in the field you're dealt a good hand here. I'd say that's not necessarily representative of most who're about to go the sub(s) route?
I wouldn't discourage anyone from throwing themselves into sub(s) usage, but I would like to tell them of a way to implement subs that deviates from small, inefficient cubes augmenting main speakers run fall range. High-passing the main speakers isn't "nonsense" but can have obvious advantages, and going with big, higher eff. subs in more numbers than one lends further, potential advantages to boot. Where it might get more complex is going active, but who says everything comes with the snap of your fingers?
Going even further with DSP's and perhaps bass management (as outlined by poster @mijostyn above) can add to integrational bliss, and so while very good results can be had without bending backwards per the more typical or popular way to integrate subs, it is to some a mere starting point going way back in their subs voyage.
It's like hitting someone on the head for going other ways, god forbid with the intention to raise the bar perhaps even further. It's not bloviating or arrogance, but simply sharing experience others might appreciate exploring for themselves. Make of it what you will.