Has Rel fallen out of favor with audiophiles?

May be "over-thinking" it a bit. The REL T or S series subs are great for music... if... properly set up and integrated.

And... yes... A DAP or EQ (e.g. DSpeaker Anti-mode 2.0, etc.) with automated frequency analysis / set up is invaluable to eliminate the room interaction and "room boom," which virtually all subs / mids produce, without using sound panels and bass traps. Makes a huge improvement in clarity and sound quality - unless you’re into hip-hop car audio.

REL's are the top choice amongst 2 channel and accurate theater reproduction. 2 REL 212's would be amazing so will the new REL No 25.
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Using a pair of T9i RELs, connected via their high level Neutrik cabling. Corner placement per REL instruction.
These are the first subs I have auditioned that I can actually stand to listen to because they integrate so seamlessly with my Dynaudio floorstanders. 
With my room gain, they measure flat to @ 22hz, but with visceral impact at 16 (I forget the measurement there).
They actually improve the imaging and lowest octave of my floorstanders, but I deviated from the REL guidance this way: I reduce the crossover point to the bottom of the Dynaudio's "flat down to" response (35hz), and increased the REL volume setting.
I am amazed at how well phasing works with the bottom octaves of the Dyn's - which is a significant problem with all other subs I have tried over the years.
Placement took a while - I used the Stereo Review test signals - but this was to get the loudest and most effective room coupling.
But with or without fine-tuning sub placement, integration with the mains was easy and surprisingly nice. Fine-tuning placement only extends the bass.
Other listeners comment that they are pleasantly surprised at the impact AND INTEGRATION of the sub. 2 channel or soundtrack.
They just augment and extend the bass, without calling attention to themselves. 

I agree with this recent review of the REL T9i
https://2y2ro62wfbas3svllg2i0s1t-wpengine.netdna-ssl.com/wp-content/uploads/2017/01/HIFI-142-Awards_...   
They certainly haven't fallen out of favor.

The adjustability of RELs, especially the older ones like mine (certain newer models dispense with some knobulation) , enables you (or, in my case, me) to easily "tune" them to the listening environment. My main speakers, although rated to 38hz or something (lame rating..they’re Silverline Preludes with 3.75" woofers) barely make a peep at 38hz. The sweet spot is around 50hz for the RELs step in. The wireless thing (Longbow?) they offer with new RELs might be a selling point for them, but REL is missing out on a pile of sales to current owners of their older stuff by not making one of these work with RELs like mine (it is, after all, all about me)…I’d buy one immediately, which would seriously free up placement options.