What are the best subwoofers to use with Magnepan 20.1s?

I think that the crossover set-up you choose will be at least as important as the sub you choose.  I use Rythmiks with small Maggies and they pair superbly IMO.  However, I use Audyssey bass management and there's no doubt in my mind that it's a big part of this successful pairing.

As to the sub itself, Hsu, SVS, and JL all make some highly regarded subs.  Some of the bigger Velodynes are also excellent.  Many folks swear by REL, too.

Recently heard a friend's 20.7s with Rel subs; outstanding. He runs the Maggies full range and adds the Rels below 80hz. He works for one of the world's top orchestras and the combo makes him & his guests very happy. I am a JL fan, but haven't heard them with planars. Cheers,
Spencer

20.1's setup properly in most rooms go down to 25Hz.

Why do you need a sub woofer for a measly 5 Hz?

What about Image size of 20.1 bass, and sub woofer bass image size?

How do you handle standing waves, peaks, and valleys?

Why bother with bass below 30 Hz, and the associated problems?

NO SUB WOOFER IS NEEDED! 

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Why do you need a sub woofer for a measly 5 Hz?

...because audiophiles want ALL of it, that's why.
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Quality of sub is not that important...just use them to correct dips around the room.