the H8000FW S/Pdif 1/2 connector can't be used for external clock.
You should use S/Pdif 3/4. Enable this connection setting DIN: 11/12 under SETUP > INPUTS menupage.
When using S/Pdif (or analog/AES-EBU/ADAT) you don't need to aggregate the H8000FW. Aggregation exclusively refers to using the FireWire driver.