So it seems that the PNY XLR8 DDR4 3200 (PC-26400, 1.2v, CL20) is at least "finicky" in this system. Known to work in the PAHi7 (one I bought for another person in my company). Known to work in at least one PAHi5 prior to a BIOS update, but not after. Doesn't work in mine. Populating the lower slot prevents the system from posting. Seems I'm going to have to RMA the kit...hopefully I can.
Anyway, the Intel compatibility list has only 4 16GB SODIMMs, 3 of which appear no longer to be available and the fourth available for about 2x market value. These aren't options.
So what makes RAM incompatible with these systems? Supporting the Intel XMP 2.0 standard shouldn't by itself disqualify RAM from working in a modern Intel system. But maybe it does.
These were recommended when I checked out at B&H (PC-26400, 1.2v, CL22): https://www.amazon.com/Crucial-2x16GB-Laptop-Memory-CT2K16G4SFRA32A/dp/B08C4X9VR5
These appear basically identical except 30% less money (PC-26400, 1.2v, CL22): https://www.amazon.com/TEAMGROUP-T-Force-Zeus-3200MHz-PC4-25600/dp/B09BQTC5QH
Neither is on the compatibility list. Neither uses the word XMP anywhere in the description. I think both have identical timings. What would make one work and the other not? Or will neither? Or both?
Thanks, though that makes me even more confused. If you dig into the product numbers, there's really only three options. Two (CT16G4SFRA32A and CT16G4SFD832A) are basically identical. Same specs (1.2v, 22-22-22 timings). Presumable differing either in layout (though the pics are literally identical) or country of manufacture. The third (BL2K16G32C16S4B) is clearly gaming RAM with totally different specs (1.35v, 16-18-18-36 timings). RGB lighting. XMP support. And a solid 1.9/5 star rating. I would have expected that at lest the 1.35v spec to be one of the key determinants of compatibility, especially since the DIMM slot on the board has 1.2v printed directly on it.
To the best of my knowledge, this is the fastest DDR4 3200 JEDEC kit there is:
You get 3200, cl20 and dual rank configuration. Its not going to matter much over more typical RAM, but this is as good as it gets.