Well, perhaps all that was needed was a reseat? The 5900X is working in the B550 system with 32GB Hynix DJR based RAM, and 3700X working in this X570 system with 32GB Samsung B-Die based ram. Doing ongoing tests on both systems, AIDA64 mem and cache test on the B550 provided the expected results for a 5900X, 62ns latency on memory, 10ns latency on L3 cache. For now I'm going to leave both processors as they are and allow more time for testing as well as seeing how things go under use? Perhaps something will show up in due course. Suffice to say, evidence so far suggests an RMA wouldn't be necessary.