Adding a sub woofer?

Longbow and Swarm...somebody will figure that out.

If I may add my inquiry here to this fresh thread, it is related to the integration of the sub and would likely be good info for the OP. I have a Hsu Research TN 1225 x 2 sub set up that I have used for 20 years. I had it set up with RCA out of my preamp with the mains running full range. I recently upgraded my mains to Wilson Sophias that have much better low extension compared to my previous speakers. What I loved about the Hsu TN model was the separate amplifier that sits on your rack with a volume knob. The bass varies greatly from album to album so I was easily able to adjust the volume up/down as needed when I put on a new record. I disconnected these subs with the Sophias and am finding myself missing that really nice low extension, especially at lower volumes. I would like to try something more modern, like the REL.  I am not sure why, but the Hsu sub did not integrate with my Sophias as well as my previous speakers.  I felt it was because they didn't extend low enough and were in competition with the Sophias.  With my old speakers, the bass was immediate once the sub was on but barely noticeable with the Sophias.  My concern is getting a sub with the controls which are not as easily accessed dialed in to work all the time. Do you find that once you get the crossover and volume set for your system on a traditional single box sub that it doesn’t require adjustments?

Do you find that once you get the crossover and volume set for your system on a traditional single box sub that it doesn’t require adjustments?

I might have a situation similar to yours. My speakers go to 20Hz but I have a standing wave in the room that cancels bass at the listening chair even though elsewhere in my house the bass threatens the plaster. To break up the standing wave I added a pair of Swarm subwoofers, asymmetrically placed in the room. Once I got everything working I did not have to make any further adjustments regardless of the album playing. With a single sub this would be harder to do since you have less ability to break up standing waves. So how much bass you get will vary from album to album.

Regardless of Millercarbon's snarky condescension (per usual), RELs are NOT inherently "lumpy" unless set up badly, and a single one can work. Not everybody has room for a Swarm of four subs and many can get along with one well placed REL or another well designed brand. And there is a big difference between high level power amp driven signal and a single ended preamp signal as ideally the sub should sound like the amp, and the high level REL style works swimmingly. For reasons noted by atmasphere and others, using two subs (I use 2 RELs) can eliminate standing wave issues and get a room in tune easily, but if you can't afford 2 subs, simply get one and add another later if you feel the need.

One subwoofer can be very frustrating to an astute listener. Best to get at least two to start. Then they are matching. If you can't afford two save longer. 
If you use an appropriate crossover they will not color the midrange at all but interestingly give you more sense of depth in the midrange. There is more air in the room. It is an effect that most people notice. Turn off the subs and everything goes flat. But, the crossover is critical and the only ones that offer total control of frequency and slope are digital. People who are married to total analog miss out on this. JL Audio makes a decent analog crossover.