What is best sub to match with Magnepan?

I use a sealed SVS sub with my -.7s.  Blends really well.   Select the sub based on room size  and add one for each channel 

I run dual SVS PC 4000s in extended mode (flat to around 16 Hz in my 25x40 open floor plan living room.

They operate in stereo mode (my processor has that option, not many do.)

I measure over 106 dB @ 17 Hz (REW w/ UMIK-1, 90-degrees) from across the room. And it sounds ’TIGHT.’

They certainly "keep up" with my Maggies.

I also own a rear 15" Earthquake that is crossed over at 20 Hz for extra low-end (with fear) when playing movies.

The Earthquake is fast, but not as fast as the PC-4000s. So they are relegated to a movie-only role.

The people who say "No subwoofers are fast enough for Magnepans," have been playing with sub-$1000.00 off-brand or REL subwoofers.

I dare those nay-sayers to order *real* SVS stereo subwoofers and proclaim they are not fast enough.

Those people clearly do not possess the budget, time, or inclination for true woofers.

And they will clearly *never* understand, hear (or feel) the other dimensions of the bottom octaves they are missing.

Nearly a decade late but:

What is best sub to match with Magnepan? 

Four.

Don’t know the best but my local dealer always ran Rel subs with Magnepans and his setups always sounded sublime.  The man had golden ears. 
I used an old M&K with mine when I had them. It was fine.