My guess is that you have to separate the issues:
1) Get the routing program working properly, meaning that the H8000 routing screens match what you see on the Routing program's display. You may still have a MIDI/serial issue here.
2) Once this is done, then worry about the Cubase. Your problem may be entirely a Cubase issue, in which case we can't help you much. It would be interesting to know why it tries to change the sample rate. Maybe the loaded project is set at some other rate ?
It's always worth re-checking the Control Panel settings, and trying to send/receive audio with the simplest possible configuration.