Not so fast, Charles. First, the language "...it will cause uneven frequency response for every seat..." doesn't appear to be supported by Dr. Toole's research or any other to my knowledge. In fact, the arrangement with one sub at each of the four mid-wall locations is one of those recommended both in the original Harman paper and the Toole book. Now, this relates to bass evenness, not bass strength. A sub in a corner is louder than a sub at mid-wall, so the mid-wall location is "inefficient" from the standpoint of level, although it can reduce unevenness caused by room modes. If the level is still adequate, this shouldn't be grounds for rejecting the mid-wall configuration.
If you read the book at pp.226-227, you'll see that a different four sub arrangement is most highly recommended from the standpoint of both level and evenness. This is a two mid-wall placement, but with two subs co-located(can be either vertically or side-by-side)at each position with drivers within a quarter wave length of each other so that they support each other with a resulting 6dB level increase. Dr. Toole states that "...it is, by a narrow margin in both MSV and overall sound level, the prime choice for a four-subwoofer solution. Either side or end midwall locations can be used. If corner locations turn out to be more practical, use them".
