Ok first, yes I KNOW that Fluid Source is deprecated but I am working on an older project and want to get working properly.

I've checked many set ups, and referenced the documentation but puzzled as to why I cannot sometimes visualize density or velocity at the SOP level from my Fluid Source.

Can anyone have a look at the attached .hip and tell me what I am missing?

You don't have enough density value. It is there, try scaling density to 10 and it appears. You also have zeroed the Out Feather Length. Try giving it a value greater than zero.

You could also scatter points into the tube. Drop down a wrangle to assign a constant or random density value for the points in the volume, before the fluid source. Change the method to Stamp Points for this to work.