ALL W90 versions have the sand-filled backs (all STOCK factory ones, that is). There's a pair of W90's that's been for sale on Ebay for months and months (the ones that look like the guy didn't bother to clean the dust off the tops), and one cabinet looks like it had a rear-panel replaced (the NON-vented side, I think), and it looks like just plywood and not a sand-filled one. Those sand-filled panels do A LOT. When you stand behind a pair of W90's while they're playing, you won't get that big pocket of bass behind them like with most speakers. The sand-filled panels do an amazing job of dampening and eliminating backwaves (or whatever they call that when you get a big pocket of bass behind your speakers). You hear nothing except the sound coming out the front. This means you don't have to worry about what's behind them, and you can put them right up close to the back wall. All of the sand-filled models are good for that..