Need some advice on the best two way Subwoofer

You need a tight fast subwoofer that will need to be able to be crossoved over at 40-60hz to get it to match well. Vandersteen subs with the sensing cables or Rel (a bit overpriced) maybe a JLAudio F112 might be fast enough (give it a listen). You also need a pretty steep crossover slope 18/24dB. The James Subwoofers are fast enough also.

You will be really happy with the results of mixing a sub with the Magnepan, it just has to be as fast as possible.

I have used a REL Stentor III, a REL Storm III (both run full range and set for 28HZ and below) and also an older Velodyne ULD-15 II with fixed cross over high pass out at 80HZ to my MG 3.6s and now 20Rs and all have been extremely satisfactory. No matter what others feel, I at least, have found that even larger Magnepans have an improved fuller sense of completeness when accompanied by a sub no matter how much power you were driving them with stand alone. This is particularly true, as you might guess, with large orchestral works and acoustic bass jazz.