Question PC won't POST with new, known working GPU ?

[rcie2017]

OK. this is a super frustrating situation, I'll try and put all the facts up here logically.

Specs:
Asrock B660M-ITX
Intel i5-12400F
2X8GB DDR4R
Thermaltake 600W SFX PSU
NVME BOOT SSD
GTX 560 GPU (placeholder)

PC works per perfectly fine.
However, obviously the GPU seems odd, thats because it's only there because the intended GPU won't post.
I bought an RX 6700 as it is clearly the best bang for buck on Australia at $450.
The initial build seemed to go OK with the first RX 6700.
New PC booted, installed Windows 11, but soon I was getting GPU errors, and the audio via HDMI was crackling and awful.
I RMA'ed that card, assuming a faulty product, but the replacement wouldn't even boot!

I took the lot to my local PC repairer, and we tried the following:

• Put new GPU in identical PC. but with older cpu/mobo (about 5 years old). confirming the card worked fine with the same PSU.
• Swapped the PSU into my case, no boot
• Updated BIOS to latest 7.02, no boot.
• Reset BIOS. no boot.
• Replaced RX 6700 with GTX560, boots fine.

The system draw is well under 600W (RX 6700 draws 175W) and posted fine with the very same PSU.
Basically, I'm stumped...why would a known-working GPU NOT work on a known working PC?
Please, can repliers avoid blaming:
Windows 11
Intel
AMD Drivers
AMD Hardware
As these types of bias-based opinions are of no help.
Thanks.

 

[helpstar]

some ideas:
check if the CPU socket pins on the motherboard are not bend or discolored
use a different display cable

how did you reset the BIOS?

 

[rcie2017]

The right-angle PCIE extender was the problem, I did a bench build plugging the RX 6700 directly into the motherboard, bingo, perfect!
Then still on the bench, used the extender, no boot!