Motu 8 pre not recognized as optical converter

[Moisturizer]

Hi, i am trying to hook up my 3 thrusty 8pre's (fw version) to an RME Raydat do to overcome the firewire driver dependancy.
The sample rates match, the signal is visible on the 8 pre's led but no signal is flowing into the Raydat.
I have two other preamps (Behringer ada8000) and they do ut just fine.
What on earth am i missing?
Shouldn't the 8 pre automatically switch to converter (and thus slave) mode when not connected via firewire?
The led says it is in converter mode.
I tried it with all 3 units i have, i switched them on and off, i even reconnected via fw so to set the clock source to the Raydat before disconnecting but no dice.
I'm sure it is my fault since the manual states this is in fact possible.
I opened a ticket with motu but since we are talking about an ancient product i have no high hopes.
Anybody here who has the experience?
Thanks in advance
Thomas