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


Hello all.  Which subwoofers would give the smoothest response/integration with Magnepan 20.1 speakers?  The rest of the system is Audio research Ref 3 preamp, Pass labs 600.5 amps, VPI ref scoutmaster turntable, Pass Labs XP 25 phonostage, UHA Tape Deck.  Thank you in advance for the help.
powerdoctor
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).
Why not try a pair of the Magnepan DWM woofers.  These are not subwoofers per se, but will add more panel area and will really smooth out that bottom end and it will seem like you added subs.

I heard a pair of the 20.7s with (2) of these and it was superb!
With the 20.1s, it would be great if you put in just one REL Gibralter, which would be much more dynamic and effortless than two cheaper subs. Not sure which size would be best--there are two.  

Save your money......apply it to 20.7s instead......

The benefits will be far outdistanced by all the issues you created.....

But if you must, I'd go with Vandersteeen