The Harman papers are well-known here and have been discussed fairly often. If you mean that you propose to put a sub under each speaker so as to create in effect full-range units, then sure, the center has to be included. Note, however, that this arrangement wouldn't achieve the benefits in room mode smoothing discussed in the Harman papers and Dr. Toole's book Sound Reproduction. That requires placing the subs at locations such as the midpoints of opposite walls or in diagonally opposite corners, not under the speakers.
