Ideally, neither XLR or RCA plugs should have a characteristic "sound". Pro audio types use XLR because of its relative immunity to RF contamination and its locking connection. In a low RF environment it's possible that single-ended RCA connections may sound better than balanced XLRs. This is due to the fact that truly balanced equipment requires a much more complex circuit design and implementation. In other words, it's easier to screw up a balanced design than a single-ended component. That's in theory, in practice it may be just the opposite. I use balanced XLRs, but then again I live in a high RF area. Your dealer is at the very least overly simplifying the issue.
RCA vs XLR
I was told by a dealer that RCA connections will tend to sound more 3 dimensional that XLR. Currently I am running everything balanced. Does anyone have an opinion and have you experimented with both formats?