Sub placement

I can see the crawl on the floor placements is similar to the speaker echo slap theory.
For speaker placement you want to place the speakers in the most responsive to echo slap.. place. Walk around the areas you want to place the speaker and clap, or make noises and listen for the area with the most echo.
That is where you want to place the speaker.
Seems to be about the same reasoning as 'crawl on the floor' theory.
As to WHY that spot is the best, I leave to your Google skills to figure out. It is after all, just one more theory among many.

Paul McGowan of PS Audio just finished a detailed blog series on this. It looked quite thorough and sensible, although I don't have a sub to try it.

You can check it out at:
http://www.pstracks.com/pauls-posts/woofer/9990/

He posts his blog once a day and the subwoofer series was done in a week or two.

Elizabeth, are you sure that you have describe the "echo-slap" theory correctly? Any room echoes are detrimental to the sound of a stereo, so why would one want to excite as many room modes as possible? Even in rooms that are perfectly symmetric only very few room modes excited by the two speakers will cancel each other, and most likely never completely.

What I typically do, is have somebody clap his/her hand in front of the midrange driver, then place acoustic panels in the room so that I have no echoes in my listening chair. This way I make sure that I minimize room reflection at my listening chair.

Regarding the link posted by Guinness, it is important to realize that there are more methods for setting up a subwoofer, and that all these methods can give good results when done properly. For example, REL advise is to cross-over the sub as low as possible, while Lyngdorf on the other hand crosses-over the sub at 300 Hz. There are also those that argue that the sub should always be crossed-over at 80 HZ or at 120Hz.

In the end, it all depends on the system, i.e. speakers, subwoofer, connection type, etc. As usually, there are more ways to skin a cat.

Try on the roof. It might work

Here is a fast method to integrate a sub into your stereo. Is it an active or passive sub?
This will work for both:
Align the sub with the woofer facing across the plain or slightly to the rear of your main speakers. If you do not have an external sub EQ such as a Velodyne SMS1, then set volume according to the average listening level of your listening session. If two subs and powered, do the same, alternate the phase on one sub to be the opposite (Note: this may be considered experimental) of the other. You will know when you have locked in the right sound when adjusting phase, usually they are set at 0, but can swing to 180.

For active subs, I really like using an external sub EQ as it works wonders.
Use something like sound anchors sound coasters under the spikes of the sub.

Finally, I am suggesting these options based on experience of using sealed subs, two 15 inch.

Good luck
Audioquest4life