If Cubase derives the clock from firewire, then your PreSonus is the master. In digital chains only one device can be the master. In this case you should look in the settings and knobs on your soundcard to see whether it is set to be the master. Now what will also be important is that any settings on the master should correspond with the project settings in Cubase. So if your master says 44.1KHz, your project should also be on 44.1KHz. Don’t mind the bit depth as long as Cubase is equal or higher. We’ll get into that another time.