Entreq users usually put one grounding box (Not the same) on each amps negative speaker output, with very strong results.
An amp and source components should never be grounded via the same grounding box unless it is designed for it (like Entreq Vibbeaters model “Poseidon”).
Amps and PS Audio power plants are usually grounded at the bottom plate, with a speaker shue fitted by a bottom screw. Power plants seems to get huge upgrades from grounding. I have yet to try my self but there are well written reviews from well known reviewers out there indicating massive performance boost.
Probably we all need both a power plant and a grounding box for most components.