Answer: It's really difficult to diagnose a plant problem without actually seeing it. You've checked everything I would have checked if I were on site so I'm really at a loss. I think your best course of action is to take a sample of one of the dying shrubs to your local Cooperative Extension office for thorough inspection. Helpful folks there can test for fungal or bacterial diseases, among other things. Contact Ohio State University Cooperative Extension at 216-397-6000.
Best wishes with your landscape!
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