Depends how complicated you want to get. Floyd Toole and Harmon suggested 4 matching subs. Earl Geddes suggests 3 subs to handle 50-150/200 Hz and one for below 50 (has to do with room modes).

I suspect if you use non matching subs there could be more EQ work to get a flat frequency response. Think about it. If you have 3 subs contributing at 100 Hz and one contributing at 40 Hz, it seems reasonable you are going to have more output at 100 Hz all else being equal.

Fred

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Blujays1: Spending Fred's money one bottle at a time, no two... Oh crap!