If you have miss-interpreted the Optical conns with CAT5, all the time, you must have had some great confusion in ya head how all that works together.
Your Q about ADI8QSM and M16..from the RME pages:
The M-16 AD offers an outstanding AD conversion to MADI and ADAT formats in a compact 19" unit with only 2U height. The balanced analog inputs based on RME's reference design from the ADI-8 QS guarantee excellent S/N and THD specs across a wide analog level range. Three different hardware levels are available. Using up-to-date converter technology, the device not only works up to 192 kHz, but also reaches a real-world signal-to-noise ratio of 116 dBA - on all channels.