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

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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.

I have had Velodyne in the past JL Audio now, both are excellent, especially if you get the models that have built in EQ (with external microphone).

The GR Research/Rythmik OB/Dipole Subwoofer.