Had to do it... Timing was perfect...lol
Did the video embed? If not.... http://www.youtube.com/watch?v=1l63PpHJ_lE&feature=share&list=UURXYFAP01iqp83P9tTl5n_Q
Yep it was perfect timing!
There are a lot of good articles on subs and placement. Many of the articles do not always take into account real world scenarios and use the perfect rectangular room as an example. Room modes exist in every room and are based off the dimensions of the room. How you place the subwoofer is going to determine how these modes get excited. Corner placement excites the modes the most and will give the biggest peaks and nulls. This often causes very uneven bass amongst the various seats in the listening area. There are advantages to corner placement though such as increased output because it is reinforced by three surfaces.
To even out some of these room modes you can place the sub in the null for the room modes. If this is done for the length and width dimensions it would be somewhere in the middle of the room and to do it for height it would have to elevated. Placing the sub in the middle of the room is not always practical, plus it is only reinforced by the floor so output is considerably less than near a wall. This is the reasons 2 or more subs are used is it allows us to get more even bass response in the listening area without placing the sub in the middle of the room. EQ out any peaks.
Since many will not use two or more subs here is some practical advice. First if possible do not place seating in a null, near the walls or corner. It is not just about sub placement but about seating placement too. Avoid bad seating locations when possible, moving the seating 6" or 1 foot can often make a difference. Don't worry about and bass areas if there is no seating there.
As far as where to place the sub? Well, it depends. Let's say someone builds a bunker for their HT and they are using some massive sub that easily outperforms their speakers. This sub is used to by the military to explode mines at a distance. Then place the sub in at least one of the nulls and try to reduce the room modes. You don't need to maximize the output. A lot of people buy HUGE subs and don't have speakers to match. Who cares if your subs go to 130db if your speakers only go to 95db. Many large subs do help with low frequency extension but it still has to be balanced with the system.
The second scenario is the interior decorator (or spouse) makes you use this tiny sub that looks like it would be too small for a chihuahua dog house. And this is going into some big great room that is open to other parts of the house. In this case placing the sub in the corner to maximize the output would be highly suggested even with though exciting all the room modes.
Now a lot of people fall somewhere in the middle and so should the sub. I'd keep the sub in the front 1/4 of the room, either on the side or front walls. Although bass frequencies are omni directional and can't be localized, the sub or other objects in the room may resonate at higher frequencies from the subs output making it localizable. There is nothing wrong with placing it at other parts of the room but these resonances often happen in average rooms and you would have to try to see if it is localizable. You'll have to measure or do the sub crawl method to find the best location for it. Avoid nulls and EQ the peaks.
You'll have to set phase and levels correctly. Be concerned with phase and level at the crossover point. Don't set crossovers too low, I doesn't matter if your speakers can go lower. It is about getting the best integration in the room. Most room this will fall between 80-100hz. By setting your crossover to 40-60hz because your speakers go lower might give worse sound. The speakers now excited the room modes because your speakers are not in a good location for bass frequencies. Plus you changed the SBIR frequency for your speakers and sub and that needs to be corrected. The crossover is not a cut off, frequencies below your crossover point will still go to the speakers. There are a lot of reasons to not set your crossover too low.
It takes a little patience to get the subs integrated well but it is worth the time.