Nexus 6 - Google splash screen boot loop after flash of 6.0 factory image (unlocked)

Hi,

I'm hoping for some suggestions regarding my boot looping Nexus 6. Here's the details:

- Brand new device.
- First boot & setup. OK.
- Unlocked bootoader. OK.
- Reboot back into 5.0. OK.
- Tried to flash 6.0, initially using flash-all.bat from shamu-mra58k-factory-5b07088c.tgz. Failed.
- Extracted zip inside shamu-mra58k-factory-5b07088c.tgz and flashed all .IMG manually.
- Phone now stuck in boot loop. Google logo appears, sits there for 5-10 seconds then reboots.

I so far have tried:

- Multiple reflashes of 6.0 factory image. Boot loop remains.
- Multiple flashes of 5.1.1 factory image. Boot loop remains.
- Tried flashing Cyanogenmod cm-12.1-20151021-NIGHTLY-shamu.zip via recovery. Flash successful. Boot loop remains.
- Flashing gpt.bin and reflash 6.0 images. Boot loop remains.

This is the first time I've encountered issues. I've successfully flashed factory images to my Nexus 4 many times over the
last couple of years.

Does anyone have any suggestions re a fix, if at all possible? Any tips/suggestions would be greatly appreciated!

If i need to return it, should I flash to 6.0 and lock? Will they be able to tell that the bootloader has
been unlocked previously?

Thanks!!