JL Gotham F212 v2 vs ?

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.
        

m-db,
Thanks for the kind words. I agree with the comments, "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?" It's about personal preference. As a matter of fact I don't use the high-level connection with my REL subs. I purchased  upgraded speakon cables and compared the high-level connection to the low-level connection. The high-level was connected via SignalCables speakon cables and the low-level via Audioquest Husky RCA subwoofer cables. After a week of listening I preferred the low-level connection...more dynamic and tighter bass. The subwoofers still disappear more than any others subs I've owned. 

The Gotham is a wonderful sub. You might consider instead getting two (or three) of their Fathom f113v2. The reason is that with subs, having several in different locations helps smooth the bass and also gives an increased sense of ambiance.

Two smaller subs also will be easier to move around, if you want to experiment.

JL Audio is a great outfit that makes a great sub. I have two f112 (v1) and they’ve made a huge difference in my systems through the years.

Rythmik F18se or G25HP in duals should be high on your list. with just a little research you'll see why.

http://http//www.rythmikaudio.com/