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

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.

Well the good news is you have the best crossover on the planet made by Nelson to fit right in with your pre and amp the XRV-1. Start with that and also I have heard the newest version of the JL's and their DARO room eq is supposed to be the cats meow according to Nyal Mellor at Acoustic Frontiers. Talk with Nyal he is one of the best acoustic guys out there for doing what you are trying to do. He knows his stuff. 

Also, I totally get the last 5 to 10 hz and the benefits of reducing your load on your main amps, etc. etc. . Read the user manual for the Pass XRV-1. Nelson talks a lot about the benefits of your pursuit. It goes well beyond the improvements of the last 5 hz.