Casey and Fred,
Yes I agree with you that if the subs are similar in performance that and they are properly placed it will probably smooth out the frequency response at each seat. But, if he is going through the expense and trouble or spending another $800 on a sub, I would think that he would want to smooth them out as much as possible.
Look at the Long Term Power Compression plots on the following links.
http://www.audioholics.com/reviews/speakers/subwoofers/lfm-1-ex-review/lfm-1-ex-measurementshttp://www.audioholics.com/reviews/speakers/subwoofers/pb12-nsd/pb12-nsd-measurementsThe Outlaw and SVS sub are two very good subs in their price ranges. The specs are also very similar. But notice how the SVS stays nearly ruler flat until it starts to reach its limit in terms out output. The Outlaw curves also look very similar to one another until it reaches its limit and compression starts to set in. The Outlaw sub has a sag in the frequency response from 20Hz to 50Hz with one port open, or a slight sag from 30Hz to 60Hz with a peak at around 25Hz that levels off as the dB's increase. Now if you integrate these two subs in the same room, then the cancellation of the nulls and peaks with be less so than if you were to just use either a pair of Outlaws or a pair of SVSs. This is simply because the frequency responses are different (somewhere between 2 to 3 dB's) for most frequencies at any dB level set.
The other thing to keep in mind is that these long term compression curves for the Outlaw and especially the SVS are very good. Many subs as they start to reach their limits will have frequency responses curves that look very different than when they were at 60dBs. This becomes probably less of an issue with having 4 subs, but may be an issue still with two subs.
