Thanks, Supermacro. That's quite helpful. That would explain why my E5-2620s would see any RAM at 1333MHz. But my E5-2670 CPUs also exhibit this behavior. Granted, these are C0 ES CPUs, but, they work at 1600MHz just fine with 8GB RDIMMs that I have.It has nothing to do with BIOS. Actually, the memory speed is controlled by your CPU so if you have, say, E5-2620 then the max you're going to have is 1333Mhz even with 1600Mhz. You will see 1600Mhz when you start to use E5-2650 and above. Also, if you have more than 2 quad rank DIMMs per channel then you might see your 1333Mhz automatically downgraded to 1066Mhz since the processor can only see 8 logical ranks.
Perhaps I should look more carefully at the similarities and differences between the RDIMMs that work and the UDIMMs that don't. Thanks again.