Who makes the best subwoofer for music? REL or JL AUDIO? REL uses High level, JL AUDIO low level with EQ. Which will be better for music. 
Try Perlisten subwoofer. from my experience D212S subwoofer is a better subwoofer than JL or REL offer - better DSP, less distortion level.

so many times you have to touch the subwoofer to actually "know" it's playing, never before i  have heard such subwoofer speakers (Sash DAW) smooth integration.
@james633 , I think you underestimate yourself. Building one of these kits is not that hard. Finishing them nicely with something like Piano Lacquer is not so easy but you can have that done by a pro for reasonable money. I tell people otherwise just to cover the enclosure with black carpet. 

The JTR woofers are very interesting. It looks like they are using a Dayton reference 18" driver which is excellent. Plywood construction is better than MDF. They are using a smaller sealed enclosure, a lot of power and DSP to force the woofer down lower. It is still a very large subwoofer. It would be impossible for me to fit four of them into the situation I have and with multiple subs a woofer that large is overkill. But using two in a point source system in a room 15 X 25' or larger would be fine if someone did not mind the look. You will also have to weight that enclosure down to keep it from shaking. I would put a granite slab on top of it. Used with digital bass management I would think it would be significantly superior to any Rel, SVS or JL sub. Here is the driver
I owned the JL 112, it was not fast enough to perfectly melt with my main speakers (Sonus Faber Serafino), sound was a bit muddy.

I have now replaced it by the REL Carbon Special and it works great. REL is way faster and that results in a cleaner sound.

Hope it helps

Get dual carbon specials and thank me later. You won't regret it; they sound INSANE. Or at the least dual 812's from REL. Much like my iphone, everything good is made in China (along with plenty of not-so-good-stuff). JLs are good as well and reach deeper, but not to my liking. No bad choices here...flip a coin...and hope it lands on the
From past experience, it’s not about how big the subwoofers need to to be. I completely understand the larger the sub, the mor air it moves. There is a trade off and it is a negative one. The base is not going to be detailed. It’s about matching the speakers you are using for your 2 channel rig. It needs to have the speed and accuracy as well. It’s easy to forget this. It’s like an elephant trying to keep up with the cheetah, lol. Big is great for movies because most soundtracks are muddy, slow and sloppy. Big subs benefit from that. Music in. A two channel rig doesn’t. I learned that from IASCA Competition in Car Audio decades ago. I applied that in today system and loving the output. Remember make sure the subwoofers complement you system, not drown it. Makes all the world of a difference.