Yeah, when buying a Martin Logan sub, go for the sealed rather than the ported ones. That's what everyone says, anyway, I have a sealed one myself so I've never heard the ported ones.
Also a big sub isn't necessarily the best choice for mating with a Maggies. People often have better results mating subs with small drivers than big ones. This probably isn't due to "speed," which isn't a factor in a woofer -- all woofers are slow. But it's a commonly-observed phenomenon. What most people want in a sub that mates with a planar is clean, flat, undistorted sound. That typically means an enclosed sub rather than a ported one, tuning for accuracy and flat response rather than extension, proper damping, a linear motor, a solid non-resonant case, and so forth.
Of course, these may not be your priorities -- if your main reason for getting a sub is to reproduce dinosaur footfalls and explosions, you may want to choose extension and output over accuracy.
I wouldn't worry about enclosure size per se. It's dependent on the technology used, e.g., ported vs. sealed. What you want to look at is the sub's frequency response.
To find the best crossover point, experiment. The bottom end extension of the 12's depends on the room, and their and your position in it. Uncorrected small room response is typically very rough, and depending on whether the low end of your 12's frequency range is in a peak or null, you could either get more bass extension, or less. The key is to find the frequency that gives a good transition, using a test signal or "walking bass." Then you can experiment with raising the crossover frequency. Taking a load off the 12's woofer will allow it to play more cleanly and increase maximum level, but you'll have to balance this against the superior clarity of planar bass. No right answer here, it's going to depend on your preferences and the levels at which you like to listen.
Another consideration is that the smaller Maggies typically have a resonant bass rise to increase their extension and establish tonal balance. With a sub, that could make your system sound bass heavy. If you cross the sub over at a higher frequency, you'll bypass the bass resonance. If you cross it over at a lower frequency, you may want to equalize it out.
Finally, if budget permits, consider using two or better yet four smaller subs. Positioned symmetrically, they can cancel room modes.
Also a big sub isn't necessarily the best choice for mating with a Maggies. People often have better results mating subs with small drivers than big ones. This probably isn't due to "speed," which isn't a factor in a woofer -- all woofers are slow. But it's a commonly-observed phenomenon. What most people want in a sub that mates with a planar is clean, flat, undistorted sound. That typically means an enclosed sub rather than a ported one, tuning for accuracy and flat response rather than extension, proper damping, a linear motor, a solid non-resonant case, and so forth.
Of course, these may not be your priorities -- if your main reason for getting a sub is to reproduce dinosaur footfalls and explosions, you may want to choose extension and output over accuracy.
I wouldn't worry about enclosure size per se. It's dependent on the technology used, e.g., ported vs. sealed. What you want to look at is the sub's frequency response.
To find the best crossover point, experiment. The bottom end extension of the 12's depends on the room, and their and your position in it. Uncorrected small room response is typically very rough, and depending on whether the low end of your 12's frequency range is in a peak or null, you could either get more bass extension, or less. The key is to find the frequency that gives a good transition, using a test signal or "walking bass." Then you can experiment with raising the crossover frequency. Taking a load off the 12's woofer will allow it to play more cleanly and increase maximum level, but you'll have to balance this against the superior clarity of planar bass. No right answer here, it's going to depend on your preferences and the levels at which you like to listen.
Another consideration is that the smaller Maggies typically have a resonant bass rise to increase their extension and establish tonal balance. With a sub, that could make your system sound bass heavy. If you cross the sub over at a higher frequency, you'll bypass the bass resonance. If you cross it over at a lower frequency, you may want to equalize it out.
Finally, if budget permits, consider using two or better yet four smaller subs. Positioned symmetrically, they can cancel room modes.