Subwoofers that do both L/R & LFE simultaneously?

Running a 2 chan/HT combo. I understand the REL subs allow you to connect both the stereo high level inputs and an LFE cable and the unit apparently senses the signal. You send the sub a full range signal. Any others do this? My Para s8's don't really need help on 2 chan, but it makes sense to me to use a REL type setup with the sub crossover at around 30. That way I get a little fill-in as needed. Am I missing something, why aren't all subs done this way. Thanks, this forum is great!