I would run away screaming from the Polks. I would encourage you to look at other brands, my pick in your budget would be Infinity Primus, try the
p163s for fronts and surrounds, and a
c351 center. They will sound better and cost less. As was mentioned before, there is no need for towers when you are getting a subwoofer, especially in such a modest budget. I would aim for a better sub too. The test results for the PB1000 that I have seen are not that impressive. Very good extension, but it just doesn't get very loud. My picks for a sub in that budget would be either the Hsu VTF2 mk4 or Rythmik LV12r, they don't cost much more but will be higher performers.
The Behringers do look interesting. They look like a great choice for a budget home theater with an acoustically transparent screen. They are an eyesore, but if you can live with that, they might be great. The problem in that system your intended subwoofer is bad match for those Behringer speakers. It only has a small fraction of the dynamic range of speakers like that, it would be like whisper in comparison. It would run into clipping and compression when the Behringers were just getting started. For a tight budget with such powerful speakers, you might think about what can be done in a DIY project. I would be looking at a
Martysub or a
Stonehenge flatpack. Throw in a
Dayton Ultimax 18 driver and a
Behringer iNuke 3000, that will have many time the performance of a PB1000 and should be able to keep up with the Behringer speakers. If you do not want to take on a project, I would look at the Reaction
PV15x, or, if you can wait a couple months, the
new Hsu subs. Neither of those will match a 18" DIY sub like the one I outlined above, but they will still dwarf the PB1000 in performance (and size).
One more thing, you asked about the difference between sealed and ported subs, to put it very simply, ported subs have a lot more output around their tuning point, but very little output below that, whereas a sealed sub won't match the efficiency of a ported sub around its tuning, but will have output below that point. Here is a chart to illustrate the difference:
In a tight budget, it makes more sense to go with ported.