If you calibrated to Gamma 2.20, and the display brightness to or about 85L or 85 candelas, you have the contrast and brightness set correctly. Double check both.
2) Viewing conditions?
Comparison conditions - the standard would be a viewing booth that is dimmed to the same brightness as the display and the comparison is made side-by-side. That is part of the industry standard. Color temps should be matched - but will not affect brightness of the print. The room should be dim to about 50lux. That's the rest of the industry standard.
We use GTI dimming viewing booths and we calibrate to 5000k because that is the color temp at which human vision is best designed to color comparison. What are your viewing conditions like?
3) Wrong Curve?
I am assuming that you are using our K7-2880-XXXXXX curves or our P2 curves and not using other curve sets by mistake?
4) Inks in the correct position? One or more switched?
Does the output otherwise look normal? Or does the output look excessively dark in the mid-tones or quarter-tones but normal in other regions?
Do you have a spectro to measure L values? You can print out a 21 step and send us measurements to verify the output is correct and there is not some unforeseen Gamma bump being done that would make the prints darker for some yet unknown reason.
Let us know if any of these may affect it.