I'm pretty sure you can use the es8 as a standalone ADAT expander - don't use the usb and connect the ADAT I/O to the ADAT I?O of your Motu... the extra channels will show up as part of the Motu in your DAW
check the manual - it's easily accessible on the ES site!
-- JimHowell1970
you know what i totally forgot it did this because i always have the usb plugged in to mine! But honestly, for use as an ADAT converter from an existing interface i would still probably go for the optx for the full 8x8 channel set...