@phusis and @atmasphere are making a lot of sense here. Most of the distortion created by subwoofers is not coming from their drivers, but from their enclosures, the room and poor integration. Using large and or multiple drivers that have well designed, well ventilated motors is the best way to eliminate distortion on the part of the drivers by minimizing excursions, keeping the driver's suspension well within its linear zone. After that it is all about enclosure design and construction, the layout of the room, the use of multiple units against walls and in corners and proper (digital) bass management. It is difficult to determine what crossover and slope are going to sound best in any given system. It is a trial and error process. You have to have an assortment of choices to find the right solution. This is next to impossible in the analog domain and no problem in the digital domain. In my current setup I tried 5 different solutions before finding the right one, 120Hz @ 84 dB/oct. both high and low pass filters. 120Hz works for me because the subs I use are clean up to 200Hz, they are stereo, taking 120Hz and under away from my main speakers, full range ESLs, cleans them up dramatically and increases headroom by at least 10dB. I get away with it because I can utilize a very steep digital filters which keeps the subs out of the midrange. I did not know this was going to work until I tried it.
There are multiple methods to integrate subwoofers. Most of them I would characterize as unfortunate. Subwoofers still have a terrible reputation in some circles because of this problem. Using analog bass management, it is easier to make a system sound worse with subwoofers than better.