Need some advice on the best two way Subwoofer

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.

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.