Thank you gendlemen for your responses.
To be honest I got rid of my v-dac recently (I ve got to update my profile) and I hear no difference with my asus being the sole source in the system.
What I am thinking is to get rid of the asus too and get the all new audiolab dac/preamp to connect it to the realtek s/pdif.
Thus, I'll end up having an analogue volume control as well, so much needed in my system consisting of a power amp and no preamp at all at the moment - the asus has a volume control in fact which I manipulate either with the computer mouse or my smart phone but I am affraid affetcs the quality as it is a digital one.
So, given that I'll hear no detectable sonic differences from the realtek s/pdif compared to asus (what you are claiming in other words), this solution looks to be the fittest and at a resonable cost.
That's my story in brief.