Probably beating a dead horse here, but has anyone of you players tried the pressure-less balls yet???
We played just today, in 43 degree weather. The PL balls were lively even after spending the cold night in the car. We play around 150 sets outdoors every year, and two 18-ball bags have been all that we needed for the season, which is fairly amazing.
I don't expect to ever go back to pressurized balls. They're dead after a set of banging from the baseline, and I always hated to create that much trash. This is better.
PS: Don't buy the Penn PL "practice balls"; they are wooden from the get-go... Oddly enough, the cheaper "Athletic Works" balls at WalMart are far more lively and durable. We can impart spin on those just as we have on pressurized balls.
Go green and try them! I read that most balls used in Northern Europe are of this ilk. And Sweden has produced some pretty good tennis players.