Most peoples don't know but there is a software calibration function in the System Prefs monitor under OS X. This is a visual procedure. You will never get results as good as with an hardware calibration device, but it's worth to try it and it will give you a reference to compare with.
System Prefs/Monitor/ click on the color pane. then on the calibrate button. Use the advance mode and follow the steps. It should give you a decent monitor profile. You will be able to switch between this profile and the Datacolor One later to compare.
Also, some iMac can manage the level of the screen brightness dynamically. There is an option in the monitor's prefs for it. You can check it or not. Be sure that this option is NOT checked (doesn't allow it). One time, I calibrated an iMac screen in a room where the light was not constant (due to clouds passing in front of the sun outside). The iMac was changing the Brightness while the screen calibration process was running. The resulting profile was not good.
It's possible that a faulty spectro is the reason for the dull colours. There is a small filter inside the spyder. On one of my spyder the filter did not stay in place correctly and then it was impossible to get a correct monitor profile.
We need more info to help you. Let us know more about your configuration and what you're doing. Which version of software you're using, which white point and aamma value are used etc.
Daniel Vézina
QC, Canada