are the subs gonna be sitting on the top or on a side? If on top i would go with some 2by 4s because those subs are heavy! But if the are gonna be on the side i would just do the dowels. The bracing is so that the box doesnt flex or, in the case of the subs on the top, bend under the weight of the subs. the more ridged the box is the better it will resonate the sound. Im not aware of any rule on where the bracing is to be placed. i would just do where you thing its needed. i would use AT LEAST 3, 1side to side, 1top to bottom, and 1 front to back.