The 80 column mode outputs RGB-I which is the same as the old IBM monitors not like analogue RGB like the Amiga outputs to the 1084.
I've got a 128D which is really nice quality and two 128s and one PSU between them (well I think it's the PSU that's the problem on the dead one).
The 1084 will work fine with the C-128's 80 column mode. I have a 1080 Amiga monitor hooked to my C-128 and it displays all modes with no problem
You're right that it's a very different signal that the Amiga sends, But Commodore originally made these monitors for all of these models.
There was a final run of 1084s that did NOT support the C-128 - but that was very late in the scheme of things C=.
