Which one to decide to be the master? I think you need to try both ways and find out which setup works best for you. In your DMT-8 manual it should give an example on setup techinques, if not then I assume that your recording to DMT-8 then bouncing over to Cubase for mixing, if so then I would have the DMT-8 be the master and Cubase slave, cuz if you would want Cubase to be in sync with DMT-8. But like I said there is only two ways (one a slave and the other a master).