Which subwoofer should I get?

Get a Elac Varro DS1000....dual opposed 10 inch drivers of a very high quality (German engineering at its finest) that would go well with the woofers on your magico. The app has a very user friendly auto eq tool that should help you integrate it satisfactoriy even if you don't have all the tools for correct sub integration.

I would suggest any of the Rythmik Audio models, where it not for the fact that the company does direct-to-consumer sales only. I'm not sure what their audition policy is, so head over to their website to at least check them out. Then go to the RA AVS Forum, to see what RA owners have to say.

REL lovers often cite that company's excellent subs as being unique in offering high-level connections on speaker binding posts. That is not true; Rythmik offers a number of different plate amps, a few featuring high-level connections. The Rythmik plate amps offer one feature the REL's don't: a continuously-variable phase control, imo indispensible. Others include variable damping, and a choice of crossover filter frequencies and slopes. And all Rythmik subs feature stop-on-a-dime transient performance, thanks to the patented Rythmik Direct Servo Feedback control of the woofers.

I've used Rel subs for 20-25 yrs and have never seen a reason to change. I'm also a drummer who has sat near the bass amp for a long time. I know what a bass guitar (or stand up) sounds like. Rel produces that sound. But more importantly, its the ease of integration and the effect the Rels have on the main speakers. The sound from the mains becomes as if you had upgraded them. It give more of all the good things you now hear from them

I haven't listened to all the subs available. But I don't need to do so because of my extensive time spent listening to and playing live music. I can heartily recommend the Rel. One sub is adequate but two subs are better. You will need to size them properly for your room  & system.

Oops, "were" it not.

I use two REL S/510 for two channel.  My mains are rated fairly flat down to 28hz, but the subs still produce a notably fuller sound. They're punchy, not boomy. A few days ago, I moved my gear to a new rack. When I turned it on to test everything, I could tell that I had forgotten to turn the subs on.  The gain is set quite low as is the crossover frequency. They blend well with my mains.

I have a larger, ported SVS sub for home theatre.  It wouldn't work well for two channel. I'm sure SVS has better subs for music. Mine shakes the house when watching movies.