http://www.soundandvisionmag.com/speakers/1852/test-bench-definitive-technology-procinema-800-home-theater-speaker-system.html
That looks like a realistic setup for these speakers. Crossover is probably set @ 100 hz, which is why the speakers do not meet their stated range (which I doubt they could meet anyhow.) in the graph. The dip shown could be minimized, but there are trade offs involved due to the limited bass extension of the 4" drivers. The dip at 1000 hz. is odd....
I am assuming that a crossover is being used when creating that graph. If not, the frequency range is worse than I would have thought for the 4" drivers.
1) set crossover at 125 hz or so, and in doing so live with the localization of some bass at sub. In this case the dip should be flattened out some.
2) set crossover at 100 hz or so, to minimize localization effects, and deal with the compromise of a dip in the response.
Your choice really. The frequency dip will not be as apparent with movies, but very apparent with music. The best way to do this is with a home theater setup DVD with test tones. You can very easily use this to discern dips in the response and tweak your system accordingly. I use the Video Essentials disc. Avia is also popular.