This is just a question that I was thinking about having read some other threads.
Can the Chauvet Professional Ovation E-910FC be used with an ETC Source 4 lens tube? For example, can I have the Ovation body with a 36 degree lens tube from a Source 4?
Yes, definitely, have done this many times. However, with some of the older, non-EDLT tubes, things get even worse than they are with a regular source 4 due to the additive color engine. In particular with that generation (I forget which it was) of really bad 36 degree tubes, it's baaad.
You will 100% want to use the included holofilm diffuser in the gobo slot, even if you're using gobos (actually, especially if you're using gobos). If you are using gobos, just put the gobo in the same gobo holder as the film on the lens side of the film. Film faces source, gobo faces lens, image looks great.
I will say that the Chauvet HD tubes are insanely good if you want sharp projections. I've even heard of people using the Chauvet HD tubes in Source 4 LED units.