Which subwoofer should I get?

Just so you know, SVS makes an $80 adapter that does allow you to use speaker level inputs if you have to or want to use that option. 

SVS SoundPath Speaker Level Subwoofer Adapter
 

I had a similar dilemma. I was matching to Revel Studio Ultima 2’s. I went back and forth between buying REL or Rythmik. I read and read, listened some, but never sub for sub with my own Revel speakers, in my own listening room.. I ended up buying two Rythmik  F12SE’s. I did get a discount when you buy two at the same time.
I’m driving them with an Odyssey Kismet Amp and Hegel pre-amp. I suggest you talk to the folks (Enrico) at Rythmik directly. One note, the guys at Rythmik tried to steer me to instead buy two E15HP2 subs. I didn’t listen. I regret not taking their advice. It’s a small footprint! 

I could have bought REL, the choice wasn't about the money. It was there appeared no good reason to spend more money for the REL’s.

BTW..today 1/2/2025, Echo Audio in Portland Oregon has two REL 810 subs for sale in very nice condition for @$3700 for the pair. If this would have been in play, I don’t know if I would have passed that deal up to buy the Rythmiks..

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.