I just asked my Kid because he did this for a living at one time. The reason for particle board is that it is more dense, and won't rattle like plywood. He says you actually want to use MDF particle board, as it is the heaviest. But that info is for systems that are using lots of watts from an amp to drive subwoofers. If you are just building a box to hold some 4 x 6's, the difference won't really be noticable.

Don