I don't know what UEFI version I have, as I can't get it to POST, much less get to the UEFI screen.

I do have both cables; both are connected; both are indeed CPU and not PCIe; both are oriented correctly, unless I've suddenly developed spatial visualization problems. I'm aware of the possibility of rotating the 4-pin by 90 degrees and have checked for that.

One thing to consider - the second 12V CPU cable (used in the 4-pin slot) doesn't have the in-line capacitor that the ones that come with my PSU do. I don't think that should materially affect things, but maybe they do?

Borrowing a higher wattage PSU from another system is perhaps possible, but I'd want to be pretty sure that it'll help to justify off-lining that.

I'm technically not attaching any storage media (really I just want to get to UEFI).

I have an X399 Taichi motherboard that has never successfully POSTed. Currently, with a minimal hardware setup (CPU, heatsink, 1 stick of RAM in A2), I get no boot progress and no debug error codes when attempting a boot from the motherboard power button (but I do get CPU fans and of course the decorative LEDs). 

Am I correct in thinking that aside from a BIOS reflash, I only have an RMA remaining to troubleshoot?

A summary of my previous troubleshooting:

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The overall symptom profile is reminiscent of a previous topic (http://forum.asrock.com/forum_posts.asp%3FTID=6211&PN=1&title=x399-taichi-wont-boot-up); that person also had only LEDs and no debug codes, but unlike me, also had no CPU fans spinning.

Prior to this issue, I at least had a C0 code; my initial research uncovered the need for both the 4+4 and the 4-pin 12V lines. I got another cable for the 4-pin line (confirmed compatibility with ATX standard diagrams and comparing pin-to-pin correspondence with a multimeter). This still didn't clear the C0 code, so I removed all component parts and inspected the CPU and socket. The pins looked ok; I did find some dust on the CPU interface, which I blew off with some compressed air.

After reseating the CPU and reassembling a minimal setup (CPU, heatsink, 1 stick of RAM in A2), I now had no debug codes (and also no fan). I then attempted a battery-removal CMOS reset, which brought back the CPU fan.

I have checked the power connectors for orientation and fit on both the MB and the PSU side. I could attempt another CPU reseat, but 1) I'd expect an error code from a bad reseat, and 2) I had a lot of trouble getting the screws to catch each time (Foxconn socket), and I don't want to do that futilely.

While the PSU is lower-rated than most TR builds, I don't plan to overclock, and I have a low-wattage GPU. My expected power draw should, in principle, fit well within the envelope of a 650W unit.

The RAM could potentially be an issue (I didn't realize how finicky TR RAM compatibility could be), but I expect an error code from an issue there, not a lack of codes.

CPU: AMD Ryzen Threadripper 1950X (16-core/32-thread)

Motherboard: ASRock X399 Taichi

Cooling: Noctua NH-U14S TR4-SP3 140mm CPU air cooler

RAM: Ballistix Sport LT 32GB Kit (16GBx2) DDR4 2400 MT/s (PC4-19200) DIMM 288-Pin BLS2K16G4D240FSC (White) (x2 Kits to equal 64 GB total)

GPU: ASUS GeForce GT 1030 2GB GDDR5 HDMI DVI Graphics Card (GT1030-2G-CSM) 

SSD: Crucial MX300 1TB SATA M.2 (2280) Internal Solid State Drive - CT1050MX300SSD4

PSU: Seasonic FOCUS Plus Series SSR-650FX 650W 80+ Gold ATX12V & EPS12V Full Modular 

Case: Corsair Carbide Series 200R Compact ATX Case