JL Gotham F212 v2 vs ?

No absolutes in audio, only preferences, but I do want to share my experience with subwoofers. I've owned the following subs: Martin Logan Depth I, HSU(can't recall the model), SVS SB15 & SB16 Ultra, Dual JL Audio F110 v2s with the JL CR-1, JL Audio F112 v2, and now dual REL Carbon limited subs. To my ears, in my system, the REL Carbon Limited subs are the only subs that have improved everything across the entire audio spectrum. The RELs totally disappear and have taken my 2-channel to the next level. 

So it sounds like Rel is the main competitor to seriously consider. I like the arguments you guys make about many more subs to smooth out modes.

It’s not just about smoothing out modes. Its about recreating the subtle social textures in greater detail. The sonic pictures that the REL helps create will take your system to another level. You could do 2 stereo stack of RELs (4×812se) and be under 12K retail.

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.