REL 12" Carbon vs JL Audio 12" Fathom

Thanks nonoise those are very interesting!
I had not come across them before!  Ken

OP, did you make a decision on REL vs JL?

Hi Krenooo6, 
I just purchased and received a pair of the REL Carbon Specials.  Very Happy  I was able to change the position and smooth out the booming issue I was having.  I set them ujp with the REL Air Ship wireless, which makes trying different positions a breeze.  Moved them away from inboard of my speakers pointing straight at me, and put them in far corners angled into the space.  Seems to give the sound more room to spread out and less immediate reflection points right back at them.  Anyway, happy with my purchase of the REL's.  Ken

Hi, this may be of interest ...
https://www.tonepublications.com/review/the-rel-carbon-limited-subwoofer/

Includes tips on sub placement.

I have a friend with two Fathoms. They are beautifully made and sound fine. He runs them with Watt/Puppies. I can not comment on the RELs.

Just a comment on using the term "fast"  in describing subwoofers. The speed of a subwoofer is noted in it's specifications. The higher the frequency a sub will operate at the "faster" it is. If a sub's high frequency limit is 300 Hz it will be just as fast at 80 Hz as a sub that has a high frequency limit of 500 Hz. The sub with the limit of 500 Hz can go faster, at 500 Hz. Two 12 " subs going at the same volume at 80 Hz are going at exactly the same speed even if one has a higher high frequency limit. What people really mean when they want "fast" is that they do not want "muddy" Muddy bass is bass that lacks detail and can be caused by a number of factors. The first and most important is the quality of the enclosure and the resonances it produces. Next is damping, the ability of the amplifier to control the woofer, then there is frequency response. A system that has a peak in the 80 to 150 Hz region and a lack of acoustic power below 80 Hz will sound muddy. Any subwoofer can have a frequency response good to 20 Hz with the microphone at 1 meter. Only larger subs (or more subs) with a lot of power and drivers with a high Xmax can project that energy into a normal sized room. Room control is required to do it really well. 

I hate muddy bass. I like my bass lean and tight. I want to feel every kick drum beat and I want to hear every single note a bass plays. I always adjust my system so that there is a 2 dB or so dip centered on 125 Hz covering about 1 octave and increasing volume from 75 Hz down so that the bass is up 5 dB at 20 Hz. This approach gives me the bass I like which is what I think most people mean when they say "fast."