But how much better do Hammond organs sound over time? None... That's how much. They just may sound a little different. But no one could tell because the subtle changes over time are indiscernible. It's just that some Hammonds sound different from others of the same models. This is more likely due to the subtle changes in wiring and components over the years.
Actually, most of the sound of Hammonds is due to how close the magnets are to the tone wheels. I was at Goff Professional near Hartford, CT a few years back and they claimed they could make a B3 sound like "Born to Be Wild" (still love that sound Goldie got) to Jimmy Smith at his mellowest simply by adjusting the magnets. Then they proved it to me on an instrument they were rebuilding.
I've had my B3 since 1965 and it still sounds the same (I have old recordings to prove it). My playing sounds different, but that's the mileage, not the years.
I have noticed that mellotrons can sound subtly different when different people play them. It probably has something to do with the way the keys are depressed. Take that samplers!
Rick