I'm trying to learn more about components used with a turntable, and how they might or might not fit with the Model 975 and Model 5000 that have set up.

When the turntable is set to use its own internal phono preamp, it makes sense to me that I connect it to AUX on the 975. If I were to use a dedicated phono stage, one that outputs at line level (is that correct?), it still makes sense that I would connect it to the AUX input.

But what if I were to introduce something like a tube preamp? What's throwing me is that they have their own volume controls, making me question whether it's correct to connect one to the 975. Would I max out the volume on the preamp and rely on the 975's volume control?

A tube phono stage and a phono preamp are the same pieces of gear and would be connected to the aux input, as you said. Many older pieces of gear have a dedicate phono stage built in and can be directly connected to a turntable. Since the availability of cheap phono stages many makers have dropped the internal stage. If you were to get a tube preamp you would connect it directly to your amps left and right channels vice going through the 975.