Grain direction on sub-woofer speaker boxes

Any thoughts on which direction to have the grain, vertical or horizontal?

These are a pair of 18" triangular sub-woofer speaker cabinets approx 20"x20"x22"x20". Going to be made from 30mm Birth-ply and milled/folded mitre joints where possible.

As this is triangular the side panels could even be 45deg downwards to the back corner if folded from the top panel.