Magnepan recommends the 10B Sub since they do not recommend splitting the high and midrange. Hence the crossover splits between the low panel and the high/midrange panel. The 10B sub gives more frequencies in this range to choose from.
I'm now getting intrigued by the Marchand XM126 tube crossover which can be had with almost any combination of XO frequencies and slopes. It has received some very nice reviews. Since my system is all tube up to the power amplifier (BAT VK-D5Se CD Player and BAT VK-50SE tube preamp) the idea of staying with tubes in all the front end gear is appealing. Many Magnepan users feel very strongly about the use of a tube front end followed by SS power amps as an ideal combination. Some even go with a tube pwr amp for the mid/highs in a bi-amped configuration.