​@JCathey  

If you use the asset online/offline feature religiously, then Reporting > Reporting Details > Total Downtime will give you the most accurate results the way MaintainX intended, but this usually requires a ton of manual interfacing so it might not be the best option for everyone.

If your site has good work order hygiene (having work requests / work orders generated as soon as the problem arises, and having the work orders closed when the issue is resolved), then you can use Reporting > Reporting Details > Time to Complete to show your total down hours. 

Regardless of the route you take, you need to set your filtering to capture only the assets you care about. You can do this in a variety of ways: categories, criticality, asset types, or even by filtering the specific assets themselves if there aren’t too many to keep track of- as long as whatever you decide is unique to the assets you want to track.

I utilize Reporting → Export Data → Asset Status and then filter for only the 4 critical pieces of equipment on our main production line. You can set that up as a regular scheduled report to be sent to yourself once a month. Hope that helps you. 
I do like your idea of connecting them into a single production line asset. But for this purpose I would only connect the critical ones. The others I might only identify to be part of the main production line via a custom field.