I found a "premium" grade birch play at a local lumber yard that deals with more specialized plywood. It was actually a full .75", domestic, and the birch skins are actually quite thick compared to HD's "Sande" brand plywood. Avoid "Sande" at all cost. It's complete garbage. HD also carries a birch panel made by Columbia Forest Products, which is better than the Sande brand but you have to be careful because sometimes these brands get "accidentally" mixed up. Look at the edge markings for the manufacturer.
Baltic Birch (I call it "Balsa" Birch) I found recently in 5'X5', was riddled with voids and around $70/sheet and from a reputable supplier to boot. Thanks, but no thanks.
For a great panel with good density, albeit a bit touchy to finish is, Doug-Fir Marine Exterior. It's typically A/B sided, but you have to know your stuff to adequately finish D-Fir to a 'furniture grade." A lot of Lowes carry this. It's notably heavier than the gimmick birch (whitewood) plywoods, which is a good thing for subwoofers or speakers.
Another great, hefty and durable panel that is actually great for painting is one used by the sign trade, known as; "MDO", or, Medium Density Overlay. It has a phenolic paper covering over D-fir cores that is great for paint and smooth surfaces. The downside being, in order to retain the benefit of the phenolic skin, mitered corner construction is a must. Olympic Panel was who made what I used, and their model of this panel was named "Signal" MDO. Many DOT signs were made from this product known for it's durability in extreme environments and it's ability to hold paint well. I can get 3/4" locally for about $64/sheet.
Honestly, for a large and powerful subwoofer, I'd be tempted to double any of the white plywoods, or better yet, a MDF core. The stuff just feels too light to me. MDF to me seems about the right density for a subwoofer and I'd be tempted to double that for the larger ported cabinets too.
