guys, thank you for the inputs!
I would like to add sth more here. I was informed that it is actually possible to connect the S/PDIF of the Virus into a Macbook Pro input.
The MBP input can take both analog or digital signal. In the second case it bypasses the internal A/D converters of the MBP's internal sound card.
I hear that the S/PDIF output of the Virus TI can give the best result. And it makes some sense. It produces digital signal anyway and by sending it through S/PDIF and not the analog outs, I just bypass the D/A converters of the Virus.
So all in all, it sounds good to try it out, if this way I can save up such a cost for a Duet for example. And the result would be sonically better as well. Because this way I avoid two convertions, one in the Virus and one in the Macbook Pro.
Please correct me if I am wrong and enlighten me.