When I play matches I find I need to use a new can every match or two. Your saying you put them through a ball machine for an entire season? You must love playing with super-flat balls. I;ve been using SetPoint Pulp Pressureless for a year now in my Spinfire and they still bounce like that day I got them. Pressurized don't last a week bouncing normally in a ball machine.
Not counting for any air loss, pressured tennis balls do not get nearly as much use in a (personal) machine, per session, as they do in a match.
If you are using 50-70 balls, and you hit up to 150-200 strokes per session, that amounts to 4-8 hits per ball.
In a single match, you are likely to hit one pressured ball 150-200 times (8-10 avg. games per set, 5-6 avg. points per game, 3 avg. hits per point, 3 sets, divided by 3 balls in a can).
Also, I measured moderately used, stale pressured vs. pressures ball bounce and found a very small difference between bounce (1 cm) and weight (2-3 grams).
I also found that Wilson (Champ.) balls retain the bounciness the longest. Followed by Babolat, Wilson US Open then by Penn Champ. and then by Dunlop ATP.
Wall Mart brand also held up very well.
(Tennis Spin Harry says that all these come from one of the three factories, so we are probably looking at about three main variants of the tennis balls, regardless of the brand)
One thing I did not and could not measure yet is the elasticity under force. i.e. a measure of elastic force under a string/frame impact at the speed of an average stroke.
I do believe that pressureless balls are harder on the arm, give what I feel is a more rubbery feel, while pressurized balls have a softer, more compressible feel, and that makes them easier on the arm at the full pace, but I do not have a physical evidence for what I am saying. (I think that many discussions that we have, in general, are more about the feel then a real physical characteristic of things.)