Loss of brightness is apparently common with CRTs - all of my VGA CRTs are now suffering from it (my Commodore monitors are all fine, interestingly). You could probably pillage a better screen from another eMac for negligible amounts of money.
EDIT: Oh, other solution: MorphOS CGX prefs allow you to set default gamma/brightness/contrast settings in software. Similar to the monitor calibration function in OSX Displays prefs. These settings are what make my aging CRT usable.
