Thank you for your response!
(also I live in Australia, so there may be some delay with my responses due to time difference)
The motherboard is fairly recent; an ASrock H270M-ITX/ac, which has 6x SATA3 ports.
I am connecting the sata cables from the motherboard to the 6SB itself.
When I take the drives (Samsung 850 EVO 1TB + LiteOn 120GB) out of the 6SB and connect them directly to the motherboard (using the same sata+power cables that were used to connect to the 6SB), the drives connect at SATA3 speeds.