Rell or JL subwoofer?

All three manufacturers make fine subs.

I have owned two JL F112 (original version) for about 15 years. During that time, they each have had to go back to the factory twice for problems. To JL’s credit, they did the fourth repair at n/c after I complained. The subs also emit mechanical hum, necessitating use of an AVA HumBuster on each.

If you are thinking of new JLs, they are better in terms of adjustment (easier to use and more capable) than the old ones. I believe JL has upped the specs of the capacitors (the point of failure), so perhaps new ones don’t require as much maintenance. I don't know if the new ones hum.

I have owned two SVS SB-3000 for a couple of years. They also are excellent subs and have plenty of adjustments -- more than my JLs. I also like the ability to adjust from their app. I have found SVS customer service excellent. Unlike my JLs, the subs do not emit any hum; they are dead quiet. They also are far more cost-effective than the JLs.

REL has many fans. What has troubled me is the firm’s insistence that speaker-level connections are superior to like-level ones, which seems like marketing nonsense to me. Yet many people love them.

You might consider a couple of SB-3000 or SB-4000 to replace your current subs.

P.S. Mine is a music-only system. No video.

As @erik_squires said, JL has the auto EQ feature, and that is nice to have. It wasn’t worth the cost to me when I wanted to add 2 more subs.

And as @soix touched on, SVS subs have an app that lets you apply up to 3 PEQ points to each sub, along with other adjustments that can help integration, such as continuous phase control. To really take advantage of the SVS features, one needs a measurement system, such as REW with a UMIK-1 or the Dayton OmniMic2 system.

Unless your preamp or processor lets you adjust phase (or delay) of the sub relative to the main, a continuously variable phase control is a must on a sub. Since you are doing movies, too, you may be using a processor that can adjust phase, and maybe even automatically.

As far as I know, the SVS software consists of the same app for the whole line. I think one of the low-end subs (SB-1000 non-Pro version) may not support it, though, as the plate amp needs the circuitry. I have not tried the SB-2000 Pro. SVS Customer Service would have the final word on all this.