There are two advantages to sealed subs: they are smaller than ported, and have a shallower rolloff for lower extension. I don't think the servo has any impact on those two things.
As far as the sealed versus ported question goes, as long as one can acommodate the size of the ported sub, I think it is generally the better design. Most people use their subs for both movies and music to some extent or another, and ported offer vastly superior output for movies, while being equally suited for music. The only reason to go sealed imo is if you have to have the small size.
Having said that, I have a sealed sub. But, it took two 15" drivers to match the output of a 12" ported sub in the same size box. But, since that was the biggest size box I could fit, the two larger drivers kept my output for movies the same, but double my mid bass output.
