@Crumb
"What about external ones designed for the 23pin port?"
I had one some years ago(latest Eyetech top of the range one with flicker fixer included) but send it back because it added artifacts(don't ask me how they were exactly, it's been a while..

in some pictures and the image was not as bright. But it did seem to be 24 bit with the advantage that you can unplug it and plug it back in any of your amigas.