So your native PIV modes are usably bright, and the native Amiga modes are dim. It does sound like a problem with the monitor. Where did the second monitor you mention come into play?
I will parrot what others have said: test the monitor on a known-good PC. Also test your Amiga video output on an Amiga monitor if you have one available.
I had been using the Amiga until recenty on a Commodore 1084S and the RGB output was good, bright and normal.
These 2 VGA monitors i have tried, well the 1st one was dumped in my basement for about 4 years and the second one has been abandoned/dumped at work for about 2 years.
I will try connecting a PC as you said to the CRT.
I did try connecting the PIV to my current 22" LCD Widescreen and without enough testing the display looked good and bright. But that monitor stays with my PC + only a CRT truely looks good on an Amiga
