I agree with damian. My room is large, and I have never felt a significant lack of bass with music or movies.
Obviously these DT speakers will not play a 20Hz tone at 120db's given a strict distortion limit. That is not what they were designed for. If someone wants more clean output at very low frequencies, that is where a separate subwoofer comes into play.
If you look at professional reviews, the Def Tech powered towers actually do extend very low in frequency response. The bp3000TL's were measured to have a -3db point at 18Hz, and the older bp2000's were measured to have a -3db point at 23Hz. There are many "quality" subwoofers that do not extend this low. In no way will a separate subwoofer be a substitute for a quality near full range loudspeaker. It is a complement, not a substitute.
We do not know all the detailed specifications, but having an additional powered woofer(s)/passive radiator(s) in the powered towered models does offer some potential advantages: high/mid/low frequency design controlled by the manufacturer, seamless blending assured, ability to relieve the midrange drivers of some of the upper bass load, possibility of relieving the main receiver/amplifier from driving the bass drivers, ability to adjust low frequency volume levels for each main channel.
Looking at the DT specifications, the powered towers and the nonpowered towers actually are very similar in size. The towers look very classy and not overpowering at all because of their slim profile, and they also take up very little floorspace.
On a side note, above it says that the bp3000TL and bp7000sc should be very similar in sound quality. On DT's webpage, it says that the bp7001sc is a significant step up in performance over even the bp3000TL. Who knows.