You need two subwoofers. You have three 7" woofers in each speaker. Their response is listed as down to 28 Hz. That is at one meter. In a normal room your are lucky to get anything below 80 Hz with gusto. Adding subwoofers will also lower distortion in the rest of the spectrum the woofers carry and sometimes even in the midrange driver depending on the crossover.
Do not bother looking at any subwoofer smaller than 12". With your speakers in a 14 X 25 foot room I would use two 15" subs. 12 X 20 feet two 12" subs will do. There is no such thing as a sub that is too large, the bigger the better.
