Ed is right on that so I stand corrected. 120hz is the best setting to have the subs low pass set to. If it goes higher then go ahead & set it higher but at 100hz you do risk some clipping.
As far as having towers with built in subs, I'm not a fan of those since most of the time the best location in the room for the mains is not the best place in the room for the low bass to come from. Having towers that have subs just seems a waste of money since a lot of the time people don't use them for the low bass. I just think of all the money spent that went to the amp/sub drivers in the towers. Bums me out.
If someone doesn't have a sub & uses the subs in the towers then that's fine & I'm sure there are some out there who love having a system without a big ol' sub in the room (specially the wives haha). But for some serious bass, a dedicated sub is needed, placed right & adjusted correctly.
I'm sure those DefTechs get down to 18hz...just not very loud. I'm sure with the subs engaged in them, 2ch music with them set to Large probably sounds great though!!
Home Theater may be another story.