Hi Farhad - When I first purchased the Mephisto, I had an Elecrocompaniet EC4.5 preamp which has a "balanced/unbalanced" switch. The switch allowed a user to go between the two inputs on the preamp very easily. At that time, I was moving to a new preamp made by Supratek which doesn't have balanced inputs. I was concerned because all my previous systems were balanced only. I didn't like or trust RCA - single ended cables. Whether it be because the XLRs fit better, looked cooler or were just different, I don't know, I just always used balanced.
To make the comparison, I installed two runs of Nordost Quattro Fil. One was balanced and the other was single ended. To make a long story short, I heard no difference in sonic character between the two interconnects. Using balanced cables produces a 6db increase in gain but...that was really all the difference I could hear.
Balanced cables are VERY good for LONG runs as they have better rejection of RFI but I don't think they sound better or worse as a general rule of thumb.
RCA cables are annoying. They either fit too tight or don't fit tight enough. XLR's are very convenient. In all honesty, I would prefer to use XLR simply because it connects better but I wouldn't expect it to make my system sound better unless I had a cable run that was longer than 3 or 4 meters (which I do not).
Now that I use all RCA cables, I can honestly say it that to me, the performance of RCA is just as good as balanced.