IMHO, this is the range where subs do their best work, smoothing the room induced impact on FR. As you go really deep in pitch (say <35hz), a good sub will be outperform almost any integrated woofer, but (again, IMHO) this has less impact on the end result - due to the lack of info in this range on most source material. A sub may provide audible improvement way down there, but the "clean up" over 50hz can be truly dramatic.
I use 12" Rythmiks. Remember that a kick drum fundamental is usually >50hz. You shouldn't worry about small drivers at 100hz. The key is carefully matching level, slope, phase, etc at the x-over point. A good sub controller (Velodyne SMS-1, etc.) will make this vastly easier.