First of all, it is false that the lsim705 have subs in them. Maybe the person who told you that has mistaken them for the LSI, which had powered side firing subwoofers built in.
Firstly, what the LSIM705 have is 2 cassini oval shaped woofers... They are woofers , not sub woofers, and while they seemingly have decent bass output, you'll need a subwoofer or two to hit the bottom octaves. Especially if you are a movie buff, you can't go without dedicated subs.
Secondly, that Polk sub is easily beatable for price/ratio quality. Polk audio subs aren't awful, but you can eaaaaaaasily maximize your bang for the buck on subs going somewhere else. I'd suggest some Internet direct subwoofer companies like SVS, Power sound audio, HSU research, Rythmik or Reaction Audio. The Polk sub you have chosen is not in the same league than the LSIM which are superb speakers.
Thirdly, if you have a powerful receiver, it should be ok. But a lot of LSIM users prefer to supply them with better power, and add a power amp to the mix. They are kind of hungry speakers, and not the most efficient at 88db/1m. Again, a power amp would be the ideal sitaution , while a powerful receiver is far from the worst case.
Fourth, it depends if your prefer directional surrounds versus "atmospherical" surrounds. The LISM703 will give you more focused, but more localizable effects, while the LSIM702 F/X will give you a better envelopping experience. Pick your poison. But keep in mind that the LSIM702s are OUTRAGEOUSLY overpriced. If you still want to go the surround FX way, you could get some Polk Audio FXI A4 AND another set of LSIM 703 ( making it 7.1 at least
) for the same price and obtain way more home cinema quality...
Hope it helped!