I'll be damned. For a few years now, I've had a problem with my MBP, and now my iMac, where videos are too dark, and I've had to watch them using VLC so I can bump up the settings.

Now I've just discovered the culprit - a setting in Universal Access called Contrast, which was turned up about a third on its scale. I've set it to 'normal', and now my screen is, for the first time on years... normal. I'd cry if I wasn't laughing.

At some point, I must have accidentally hit the magic key combo, which is cmd-opt-ctrl-. (right next door to my customised 'Empty the Dustbin' key combo of cmd-opt-ctrl-/ of course) - and all the time I've been trying to fix the display calibration, and all kinds of other arcane settings.
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If it's brokenless, don't suffix it...