Which subwoofer should I get?

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.

I could have bought REL, the choice wasn't about the money. It was there appeared no good reason to spend more money on the the RELs. 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 it I would have passed that deal up to buy the Rythmiks..

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+.