Mmmmmm.... It´s not exactly perfect, but at least an option. Thank you for your support!
We have our first "digital" live gig next Saturday with a Windows 8.1 64bit machine, 16GB RAM, Sonar X3e Producer, a RME RayDAT card, two 64-Channel Appsys ADAT Multicore Extenders, modified Behringer ADA8000´s on stage and the Appsys Remote Gain Controller as well.
So far it runs very stable, at least during practicing. The fantastic RME ASIO driver is configured with 64 samples buffer and we use 18 input channels and 6 output channels for now. All that with less than 6ms (!!!) overall latency. It´s amazing! No problem of any kind so far.
Hopefully the gig goes as well... ;-) I will let you know how it went.
Andy