I have been developing a replacement for the NeXT monitor and/or soundboard for a while now because the monochrome systems rely on an N4000, N4000A, or N4000B monitor. You can get around the issue by having a non-ADB soundbox and building a video converter, but the non-ADB soundboxes are hard to find too, and older models can’t use ADB versions… and even if they could, the ADB systems weren’t fully compatible with a lot of the NeXT software.
The ultimate goal is to build an adapter that will plug into the DB19 monitor cable, and which will provide VGA/DVI and PS/2 or USB keyboard and mouse support. Wouldn’t it be fun to have a NeXT cube connected to my KVM? Given that the monitor bus protocol isn’t really documented, and what information is available is inaccurate, this will be a fun project.