Subs for

I would use a REL or similar and cross them as low as possible. A friend of mine is trying out one I bought recently with Quad 57s. He has gradually lowered the crossover point to 25Hz, which is the point I use with my two RELs with Spendor SP1s. I run my main speakers full range as I can hear a degradation when running through the crossover, I think you would be able to hear it through the Maggies also. But you can try it both ways and see for yourself. I would not run them above 30 Hz but again you can try it yourself. The RELs run off the amplifier power output as well as line level, this is the method I have described. If you do not have an output that is controlled by the amps volume control it is hard to use line level output to drive the subs.

My experience with a sub when I had Maggie 1.6's was rather different than Stanwals. (Quite a nice system there Stanwal BTW.) I needed very little reinforcement from a sub in the room I was in, just a very slight lift in the lower registers. Following advice from those who felt it best to keep the subs in the same relative plane as the main speakers, I was constrained by lack of space and settled on a small Sunfire Jr. sub. I was able to keep the sub integrated well with a 55-60 hz. crossover point as long as I was judicious about sub volume. ( I used a db. meter to set up the sub as well.) I would agree that space allowing, the REL product line is a good choice for your application. In the end, I'd think that your room's acoustics will dictate your crossover point and anyone else's experience is of limited value.

The Thiel sub is about as good as one can get these days.
The crossove point will be room dependant to some degree--in terms of trying for a 'seamless blend'beyond thinking of just the obvious roll off of the panel--plus, I'd check phase since you're matching with a bi polar speaker. Don't be dogmatic in your assumptions, just experiment with frequency points as well as phase. Your ears will know.
Windell Diller at Magnepan should be more than helpful.

Check out the Rythmik Audio Servo Subs. They are very fast and accurate. You can pick one up direct from them or through Ascend Acoustics. I use it with my Maggies.

L

Lower x-over = better.
I use a HSU, ported sub and set it up at about 40->45hz.
I experimented for a couple weeks after getting the sub, going to lower and lower x-over, while adjusting the gain and phase.
Maybe I didn't go low enough in the x-over. I think it might be time for another round of experiments!