Just to take this in a somewhat different direction, how many prints per month are being made on that machine?
The MP 6500 is meant to be used for large jobs, if you aren't making enough prints per job per month the toner feed doesn't run long enough to keep up, resulting in prints with poor density.
On older machines, decreasing the value for toner feed time in service mode helped sometimes (that makes the toner feed system run longer for some reason), but there's a chance you just have a machine that is too large for your needs.
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