DT doesn't get a lot of love on this forum.
One of the main reasons is that their published Frequency Response numbers are completely bogus!!!
For movies, yeah another external sub is probably a good idea. With the built in subs on the DTs, 1 external sub should suffice. If you're not watching a lot of movies, then you probably don't even need a sub.
I hope you got the deal on the DT setup? It was something like buy the front 3 and get the surrounds for free, but dunno how long they ran that special offer.
Personally, I own the DT SM350 and it is a great speaker for the $200 I paid for my pair. I also own the Goldenear Triton 2, in many ways that is an evolution of the DT ST design.
But, but, but.....
If you do get a sub, there are MUCH better options than DT out there for the same money!!!