I thought it might be time for another thread
This is a good diagram from Prince that explains a few basic things.
Granted, because of limited space, many directly daisy chain hydraulic valves using the output to feed downstream rather than using power beyond to feed the next valve block. You can see though that a sharp increase in circuit pressure could exceed the upstream valve's max pressure ( for example, running into something that then transmits a high load to a hydraulic cylinder). In this case the relief valve of the upstream block won't work since it would be a the ( overloaded) circuit pressure with nowhere to vent.
You can see how this would apply to standard double acting hydraulic control valves like most people are adding to their tractors. Most of these valves have a PB option.
What about Electro-hydraulic SCV's (EH SCV's)? For reference, EH SCV's are mounted to a manifold. In the 3000/ 4000 series it would likely be a D03 manifold.
The D03 manifold does not have a power beyond output. Can you daisy chain D03 manifolds with their respective SCV's on them? EH SCV's are actually pilot valve operated so does this change the way you can plumb them?
The reason I ask is because if you can then you should be able to daisy chain the 3rd SCV without worrying about violating some basic principle and risking ruining your valves. That would provide a simple way to add EH SCV's to the 3000/ 4000 tractors. Otherwise, yes you have to use the PB loop.