Actually this is what REL said about the mode selector. Looks like either position 3 or 4 will do...
"Thank you for contacting us, and for supporting REL Acoustics. You are correct that the Mode switch/dial on your Britannia B2 functions as the subwoofer's phase control, while also selecting whether the RCA inputs feed through the subwoofer's crossover filter circuit ("line" source, meaning positions 1 and 3 on the switch) or bypass the crossover filter circuit ("LFE" source, positions 2 and 4 on the switch). If you are only connecting the B2 using it's High Level input then you will not need to worry about how the RCA inputs function, and will only need to treat the Mode switch as a phase control. In that case positions 1 and 2 of the switch will represent a 0 degree phase setting, and positions 3 and 4 will represent a 180 degree phase reversal. "