Great advice! Thanks
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The method you describe is certainly a useful starting point for finding the best placement for a subwoofer. It does not, however, guarantee the best results. The initial placement may result in the most perceived volume from the subwoofer, but not necessarily the flattest / most accurate response. If you are using the sub strictly for LFE reinforcement of DVD soundtracks, then the method you describe will probably suffice. The ".1" LFE soundtracks usually have a broad rise between 30 and 40 Hz, with relatively little output below 30 Hz.
Music reproduction, on the other hand, should have a flat frequency response the extends into the low-20 Hz range.
So, if you also plan to use the sub for music reproduction, some further tweaking with placement may be necessary.
All of this is complicated further if you have 2 or more subs. In cases of dual subs, the room corners are usually offer the best placement spots, since you will get the most "gain" from the room boundaries, and minimize phase problems.
To really ensure the most accurate and musical placement of a sub (or pairs of subs), you can start with the placement method you've described, but it's also an excellent idea to use a test disc (with low-frequency tone sweeps) and a sound pressure meter (such as the one sold by RadioShack for $40) to see if you have peaks and valleys in the response curve. You can then move the sub(s) around a bit to flatten the in-room response -- which can often change quite a bit with relatively little movement of the subwoofer.
I find it works best when I put the sub in my normal listening postion, leave the room, walk to the pub, drink several pints and then return to the room and sit just about anywhere.
Seriously that is good advice you were given. I ended up placing a single sub according to the Cardas method for main speaker placement, next to the right main speaker. It looks a bit odd but it sounds good.