I tested the GIGABYTE Radeon HD6990 with the factory default BIOS (BIOS1 - Position 2) in my X1000 and in my X5000/40 today.
X5000:
U-Boot can't detect the HD6990. I have seen via serial connection, that the U-Boot loads and the kernel boots. The kernel detects the card but can't configure it. There are a lot of DRM kernel dump messages.
X1000:
CFE works but there are some error messages about unknown x86 opcodes. The kernel boots but it can't find any disk drives (USB, SATA ..) anymore.
Result: I cannot recommend buying an HD6990. The best card for Linux PPC on our AmigaOnes is the HD6970.
Pictures:







Cheers,
Christian