Using the Pro Channel... On a bus in X3D only way to get to a PC76 compressor is to right click an PC4K module and go to Replace Module (basically it's hidden). With tracks you can just add it like any normal Pro Channel module.

Any ideas why?
 
Thanks...

probably because it is seen as a track module .  Sure you could use anything on a bus, but they probably figure most won't .

If the PC4K S-Type Bus Compressor doesn't already exist on the bus, you can add the PC76 U-type directly just as you would on a track. Once one of those exists on either a track or bus, you can only replace it with the other type as they are mutually exclusive.

Hi Alex,
 
It's the same with if the 76 was already up. You'd need to replace it with the 4k bus. The reason being, you can't use both at the same time. It's one or the other. Remove either one to where there is no compressor and you will see them show up as "add". Using both at the same time wouldn't be a good idea so this is why you have the choice to do either or. It's been exactly the same since X1 and just to be sure, I just tried it from X1 to X3D.
 
This is not only true for buses but for tracks as well. So it is my belief that this was intended behavior. Maybe a baker will chime in to let you know for sure, but that's my best take on it for what it's worth. :)
 
-Danny

Same is true for a tracks. Only one instance of the PC76 U-type or PC4K S-Type Bus compressor is permitted.

Right I was being dumb :) ... Thanks at least this was straightforward...!