Picasso IV brightness setting??

[LoadWB]

FWIW, I ran into that problem before.  It was because the output of the PIV was out-of-range of the capabilities of the monitor.  Now, whether it was the refresh rate/resolution or the position of the screen in the output, I do not recall.

Does the screen output center perectly on your CRT?  Have you tried the lowest resolution setting and/or fiddled with the output settings?  I use P96, so I use the P96 prefs program.  If you do fiddle, make notes.

[LoadWB]

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.