The short answer is; You have to balance work done with flat panels. You wont have to balance the inside of a completed box when veneering the outside.
When you veneer a flat panel that is planned to be cut after veneering. You have warp problems that even two inches of core material can bend or curl. If you are making your own 'panels' that your cutting and machining to build a cabinet, door or other case goods. You have to veneer both sides so you have a balanced panel. Its remarkable how much a panel will bow when unbalanced.
When you veneer a completed cabinet you wont have these problems. All the panels (top, bottom, sides, etc) are locked down at the corners and the internal bracing has created a honey comb effect to hold the mid areas of the panel in place. I have never seen any panel movement on even the largest (lets say 2'x4') side of a cabinet that has been correctly braced.
The warp is actually working in your favor as a speaker cabinet builder. The typical movement expected is for the edges to curl upward. This can not happen on a box because your edges are immobile being glued, nailed and screwed in place to the contiguous panels. Since the edges can not warp up, the center will be forced to a inward internal stress. This internal stress is now putting a preload on cabinet bracing. It's pretty cool how everything works together.