Are 8 MemTest86 errors enough to confirm bad RAM?

Eight errors are enough to show that the memory test isn't running reliably. You don't need hundreds of errors for it to matter, and even an occasional error could fit with intermittent crashes.

That doesn't prove which component is responsible, though. A RAM stick is a likely suspect, but the slot, motherboard, CPU memory controller, or settings could also be involved.

Since XMP is already off, I'd test one stick at a time with the same settings. Then try a stick that produces errors in another appropriate slot. Check the motherboard manual for the recommended slots.

If errors consistently follow one stick across slots, that points toward the stick. If both sticks fail in the same slot but pass elsewhere, look more closely at the slot or motherboard. A pass on its own doesn't guarantee a part is good, so repeat tests when the results are inconsistent.

I wouldn't start tweaking voltages to cover up errors at the current settings. Check that the BIOS is current, load its default settings, and retest. If one module keeps failing at defaults, contact the seller or manufacturer about a return or warranty claim.

The power supply could be another source of instability, but this test result doesn't diagnose the PSU. I'd isolate the memory and slots first rather than replacing the supply based on these errors alone.