Which subwoofer should I get?

I can recommend 2 SVS subs depending on your budget.  The 45 day buy and try and return for free in the US allows you to make sure they sound good with your system in your room.  The most important part or buying without regret in my experience.  Their customer service is excellent after the sale.  You can talk with someone live to help troubleshoot any issues.  The app makes it so easy to use.  I run with my 2 SB1000 Pros with my iPad and it’s working well for me. 

Especially as you want the app and other features and a sealed sub, check out the Arendal 1723 1s or 2s, as its sound characteristics is said to be between the SVS and REL.  Many reviews on YouTube, especially Nemo Propaganda as he covers all the subs that have been discussed here.

I use Arendal’s smallest sub in my bedroom system paired with Magnepan LRS+.

I have a few thoughts as a sub user for 20+ years now. I have heard the brands you are discussing and many others. I am firmly in the camp that all systems improve with prosper implementation of subs. 

short answer, will the SVS sound as good as the Rel and cost less?… yep… and a big chance the SVS will sound better as it is tunable.  -6db @ 20HZ Is comical for a sub as the sub still needs room gain to get flat. At which point you are hoping for the perfect amount of room gain. The SVS will allow you to adjust for room gain and a bit of boundary interference.

Rel’s high level input is a joke. No one with a lot of subwoofer experience would choose this connection. Yes it can work fine but why take the time to dial this very old method in. 
 

 In general I would buy the biggest sub you can, think 15” or 18” subs. Each drop in octave takes 4 times the displacement. Your mains have 3x9” drivers with a surface area of 189 square inches per speaker and I bet they have good throw distance too. A 15” subs is 175in/sq and an 18” subs is 254”in/sq.  The less the sub has the work the better it will sound. You have top notch speakers that need top level subs imo.

if you like SVS I would look at the new Ultra 17 ($2500). They have a 30 or 60 day home trial.

If it were my money I would be looking at Rythmic 18 or JTR RS1 with an active crossover. I own the JTRs and have written about them if you want to see my detailed opinion. Big subs that play a wide bandwidth (10hz-200hz) will play 20-80hz with ease. Highpassing the main would take it up another level too. 

@james633 --

+1

It’s telling seeing main speakers with bigger displacement area in their woofer section than the subs that augment them. Indeed, physically subs are quite often smaller than the mains, and knowing Hofmann’s Iron Law we know what it affects (i.e.: sensitivity) - which is not without consequences. Somewhere along the line the dominating narrative has crept in that a physically smaller sub cabinet volume makes it easier to control resonances, effectively setting an upper limit for sub size, but seeing the forest for the trees (and knowing what it’s really about here) it’s also an argument that conveniently supports audiophiles’ resentment towards large sized subs for no other reason really than the inconvenience of size.

Enclosure resonances aren’t trivial, but the question is to which degree compared to lessening the workload of the woofers; smaller/fewer woofers working nearer their limit will effectively negate the work that has gone into making the cabs rigid due to large amounts of mechanical distortion from the driver itself, so it comes down to knowing the difference ease of reproduction (from prodigious air radiation area) a good sub design can have, from cabs that are as rigid as it requires for them not to impede too severely with the overall quality of the bass (hint: it can be done without resorting to builds that weighs in excess of several hundred pounds). For those wondering about the sufficiency of what’s required to make large cabs rigid enough, as an outset at least it only matters that such sub designs produce cleaner, more palpable and effortless bass than their smaller or less efficient brethren.

If one were to abide by the "no resonances"-approach of sub cabs it follows that their weight (and price) would become the actual, practical hindrance way before the woofer size/effective air radiation area would get to a place where it really mattered. Small subs, both with regard to driver and cab size, can be made to sound well with bigger main speakers within morerate SPL’s, no doubt, but that way (unless in multiples, and of course taking for granted good implementation/distribution) one isn’t taking the fuller advantage of what subs can do and how they could complement the overall sound. Shooting for energy linearity into the subs region and seeing what’s required for this to come true is a good starting point, but it’s a rudimentary physical aspect few seem to consider or won’t practically accommodate, so there we are.

my guess would be that, REL subs set ups are such an ease, the use of an APP would be frivolous.

Only Rel subs defied physics and set up with ease in spite of a severely lazy lack of features and integration tools??

"such an ease", Rel marketing is quite funny..