Well to your last question, yes subwoofer placement makes a difference. It’s actually the biggest influence on its performance.
My personal opinion is that for a room like that, get the single best sub that you can afford, and immediately budget for a second one. Obviously there’s a lot of gray area here. In that case, a pb16 in the corner is where I would go. That is however, my personal opinion and there are definitely advocates of near field placement which can have advantages but a lot of rooms don’t allow for it. Ideally you would have your LP away from the wall for better/smoother bass response overall, and this would also allow room behind the couch to place a pair of sb2k’s. Depending on what your performance goals are vs waf vs aesthetics etc, that might be the way to go.
Personally, if it were my space and had the budget, I’d go for the 16. To me it’s performance will be better than pair of sb2k. It has dsp, better extension, output and a sweet app to help integrate it. Other options would be 1 or 2 Rythmik fv15hp’s or HSU mk2’s.
http://rythmikaudio.com/FV15HP.html
http://www.hsuresearch.com/subwoofers.html