Best Big Sub? or duals?

REL is very good. I have just what you need for HT. A pair of Nelson Reed 1204s. They were designed by a California company for the movie industry to play back soundtracks at full volume in large spaces. They will reproduce 20 Hz at 120db levels. They use 4 12" Volt drivers on each channel, 2 forward facing and two facing back. The cabinets are large but not huge. I have the electronic crossover for them. When I used them on the cannons in the 1812 it would scare you. They were only used for a year or two before I moved into a smaller house and packed them up , where they have been the last 14 years. They were reviewed in Stereophile in the late 80s. I will sell them for$2900 with crossover, which is a fraction of the cost of building something like this now. The freight will be considerable , as they weigh 110lb each. I had intended to keep them but my present listening room will not hold them and I do not contemplate moving. There is info on them on line.

JL Audio Fathom F113 or 112

Parts-Express has the Titanic subs and a few of them use the Dayton Reference drivers.These probably aren't necessary ,due to being used strictly for HT.I use 2 subs,each with two of the Reference 10" and the IB woofers for LFE.They have on sale the D-dub kit for $320.These have 15" woofers mated to 240W plate amps.The Titanics are about $750 each,though they do have 1000W and a parametric EQ.

Is that the Nelson-Reed top of the line where he used ATC midrange in the mains and eight Volt 12" woofers in a pair of subs? I believe he claimed 120 db SPL at negligible distortion...WOW.

I use a Bag End Infrasub 18. Very nice reviews. Wish i had room for two so i could have one next to each speaker. Excellent IMHO. Don't know if this is out of your price range but i saw a very nice one used a week or so ago for around $900. A very unique and great sounding sub. Good luck on the down low.