Hi Paul,
Are you using a completely colour managed workflow?. Do you have custom profiles for the papers you are printing on? Are you able to print the same image from your PC and the Mac and if so, are they different? If they are I'd look carefully at the colour settings you have in your apps and see how they compare.
As far as I know, when you have your monitor calibrated (possibly your camera(s) also, the idea is consistency in colour accuracy all the way through your workflow. The brightness you talk about may simply be the brightness of the monitor setting. If you use Photoshop, select the info panel and then move the cursor over part of an image and see the actual colour values. Now increase or decrease the brightness of your monitor by using the controls on the keyboard. You will see that the actual colour values do not change so arguably, the brightness of your monitor should not impact your prints.
I've just acquired 5K iMac and am going to have to go through this calibration process myself again. On previous macs I've been able to produce accurate prints by calibrating my camera (5DIII) using X-rite Passport, then a hardware calibrator for the monitor, and finally have custom profiles made for the papers/ink that I use. Any colour adjustment required I do in Photoshop.