There is no low end comparison between Def Tech powered towers and stand alone subs like the Velodyne HGS-18. Def Tech's rated responses are a joke, they always have been. Any objective measurements of their response at the rated frequencies makes it clear that they don't stand up down low to stand alone subs. Unfortunately, you wouldn't think so from their marketing. You will still need a stand alone sub to get the very lowest of frequencies reproduced with any authority if you have a moderate to large room (if you don't then their flagship towers are probably too big anyway). The towers simply don't have the internal volume to do low bass really well.
You may be able to cross them over to a sub at a lower frequency than non-powered towers, and if that's your goal, then great. Otherwise, I'd look for non-powered towers and use the money you save on a great sub like the Velodyne or an SVS, Adire, ACI, etc.
You may be able to cross them over to a sub at a lower frequency than non-powered towers, and if that's your goal, then great. Otherwise, I'd look for non-powered towers and use the money you save on a great sub like the Velodyne or an SVS, Adire, ACI, etc.