I've been through this in the last couple of years, but on a smaller scale. I had a family member who replaced a 6620 and a 7700 with a 9510 about three years ago, and it cuts as much as the two of them put together ever could. I realize that the difference in the size of the machines involved may not make that seem like much of a comparison to your situation. Try looking at it this way: A 9650 STS should have in the neighborhood of 25% more capacity than your 9600. Add in the availability of one more truck or tractor driver, and subtract the time needed to maintain and repair one combine, and see if that is enough to make up for the capacity of the 7720. I'd guess that it probably is.