I have an I Depth, the little brother of the Descent. It is a very, very musical Sub and has a lot of adjustments to tailor the sound. It is pretty good for movies, the Descent should be significantly better. I don't think i am going out on a limb here saying that it should definitely be a significant step up from your Cambridge Soundworks Newton P300HD. I think the Seaton will be better for movies, but it is much bigger, not as attractive to look at (IMO). If the Descent is as good as an SVS PB2-plus, then I think you will like what you hear and will have significant output (just less than the Seaton).
What finish Descent are you thinking about?
ELvis has left the building!