Sharp MX2300 printing colours really dark

I visited this machine today and it was printing very dark, i cleaned and checked everything and re-calibrated the machine. the quality is actually really good but it is really dark. The strange thing is the internal calibration pages and test prints from the machine all look ok, but when the customer sends a print job (from any application) the print comes out really dark.

Any ideas guys? (they are using PCL6 driver, would a PCL5 driver make any difference?