Asymmetrically?

In all likelihood, not placed symmetrically around the listening axis. In other words, if you have a 20 x 30 room, don't place them on the front wall each 2' off the wall. That would be a symmetric placement... Can't speak for their theories, however. Give it a listen and see if it makes a difference.

"give a listen and see if it makes a difference"----yes, and I agree.

If the main speakers (with their own subwoofers) are placed symmetrically why should the external subs be positioned asymmetrically?

I own two subs, each is positioned directly behind the corresponding front main speaker and (if properly dialed-in) there are no "audible" standing waves. (Forgive me if the latter is not acurate but, I am assuming a standing wave would create boomines in the room. Please correct me if I am misinformed.)