Placement is definitely the most important thing. Failing to utilize your subwoofer properly can make big differences in sound and even cause slight vibrations in the wrong places. Avoiding windows is a no-brainer for subs and speakers alike. Corners work well, but the subwoofer must be given proper breathing room. Another important thing which wasn't listed on the poll is proper calibration and set-up of the subwoofer. The placement plays an important part (best achieved by the "crawl on the floor with sub in seating position" method), but so does the calibration. While auto EQs in receivers such as the Pioner VSX-1015TX will help most situations, setup begins from the listener. If the speakers and the sub don't match in voice or wood (I don't care for aesthetics, I'm talking strictly about sound), it could make or break your listening experience. Start with the hardware first, then look at the software. While some people would prefer two subwoofers, I feel one is good enough if it is tuned properly and not something extremely budget e.g. Dayton. However, in my preference, I would definitely go with two *grin*.
Cheers,
Raphael