Harman PDF Presentation (pdf)

One on One Technical Products Easy Room Mode Calculator

Low-Frequency Optimization Using Multiple Subwoofers (pdf)

The above link to the Welti paper is not to the AES website perhaps this would be better:

Earl Geddes Why Multiple Subs


Suggested Reading:

Master Handbook of Acoustics
Everest, Pohlman ISBN-13: 978-0071841047

Sound Reproduction: The Acoustics and Psychoacoustics of Loudspeakers and Rooms
Floyd Toole ISBN-13: 978-1138921368

Mastering Audio: The Art and the Science
Bob Katz ISBN-13: 978-0240818962