JL Gotham F212 v2 vs ?

My only in home experience with JL Audio was when a friend brought his F113 v1 ARO over and another brought his REL Studio III to compare to my Velodyne DD 18. 
I set one of the Velodyne's presets back to factory settings to compare the ARO to Auto EQ. The differences weren't dramatic but noticeable and we all preferred the Auto EQ.
The JL lacks manual EQ adjustments and the Velodyne offered manual adjustment of eight parameters. Feeding the JL from the manually adjusted DD 18 finessed the JL's output almost indistinguishable from the larger Velodyne. Very impressive for a 13" driver. Great sub. 
I'd suggest reading up on JL's CR-1 and Mini DSP.

Following the beautifully made $8900. British REL Studio III's setup procedures, the sub provided a poorly integrated, muddy presentation with a low frequency limitation.
We then relocated it, placed on its side to directly radiate the drivers and fed the same EQ'd low level (RCA) signal from th Velodyne. While still limited the REL was clearly better integrated and the equalization allowed greater output. Unfortunately not enough, its owner sold it the following week.
Looking at the suggested S/812 it's another stylish looking sub. Surprisingly, the setup procedure is almost word for word the same as it was for the Studio III.     

What about psa s7201? It looks insane but apparently delivers high quality bass. Especially since it's sealed. 

@smodtactical,

The key differentiator between REL subs and any other sub on the market is it’s high level input connectivity. By using the high level cable (included with sub), you connect REL sub from your amplifier’s or receiver’s speaker terminals. It allows RELs to effect a transparent connection to your system that permits utterly seamless blending of main-speaker-to-subwoofer. When properly adjusted, REL will transform your system with dynamics, warmth and body. And not to mention it do so with greater delicacy than you probably ever heard out of your system. None of that bloated, flabby or often overpowering bass you can’t tame from the other subs.

I suggest you read up on this more before making a decision.

Spacial not social***

Following the beautifully made $8900. British REL Studio III's setup procedures... We then relocated it, placed on its side to directly radiate the drivers and fed the same EQ'd low level (RCA) signal from th Velodyne

@m-db 
Except that you didn't also connect with the high level connection which also needs to be dialed in. The high level connection is there to support the bass frequencies your mains are not capable of producing. It's done through the Amp and not the pre-amp to preserve and better match the sonic characters also fed directly to your main speakers.

cjazzy007  We initially connected our friends Studio III using the high (speaker) level to an Ayre V-1xe amplifier and located the sub in a corner exactly as the manual suggests. 
The signal must first pass through the preamplifiers input and output sections to get to the amplifiers own input output sections were the signal absolutly took on the sonic characteristics of the amplifier.
As good as the Ayer was all three of us found the preamplifiers low level connection superior even before the Velodyne was inserted and equalization was applied.
The corner location created monstrous bass modes in that room. The REL proprietary high level cable was too short for the rooms crawl tested location. 

ricred1 has refreshingly offered an impressive list of subwoofers he's actually used along with his current preference. I find comparative first hand in home experience and opinion regardless of personal taste can be the most useful information in this hobby. Well done.
Not your typical, 'I bought one and its great'...compared to what?

REL is one of the early manufacturers to offer low frequency augmentation to the High-Fi market at a time when preamplifier section outputs on receivers of the day were non existent. Their high level connection was a very welcome method to the majority of music lovers and is still a popular choice.  

There's no right or wrong, "we may exercise more agency, assembling a system that presents the music just the way we like it, and in so doing, participate in the act of creation?" Jim Austin, As We See It, Stereophile Volume 43 Number 4, April 2020.