When compared to the Rava the Dharman will give you a fair bit better extension, and higher spl's overall. According to Tom Nousaine's numbers the Dharman gets 96dB's at 20hz, which is 3dB's better than the VTF-2 at 20hz. The SVS 20-39cs got 91dB's at 20hz but will give you better spl's at higher frequencies. Keep in mind that the SVS and I believe the HSU have gone through driver updates and will both get better numbers when tested now (the SVS was tested years ago). Also, those numbers are measured at 10%THD which would be maximum output at acceptable distortion. I am not sure what type of amp the SVS was tested with (the cs is passive), but I imagine the PCi series would be similar in frequency response.
If you have the money to spare the Dharman would probably be a nice improvement over the Rava. Remember that like the HSU the Dharman is ported, which makes placement slightly more of an issue. The Rava is probably the best bang for your buck at $399, but you do get a fairly linear increase in performance when going to the VTF-2 and then the Dharman. Whether or not it's worth your money depends on how loud you like to play movies, how big your room is, and whether or not the extra extension is important to you.
Scott, don't forget about the Adire Daeva. It combines the output of the Dharman but it's in a sealed enclosure like the Rava. Supposed to a very impressive sub. I think that Adire is offering a really good introductory price on the Daeva as well.