Balanced vs. Unbalanced - What does it mean?

True diff. balanced circuits use "common mode rejection" to reduce the noise floor of the equipment and increase signal to noise ratio. (not necesarly just line noise and RFI)

Some (many) components have XLR connections tacked on the back, but the circuits are not true diff balance designs and many times they may actually sound better just using the RCA jacks because the signal path is actually shorter.

Generally, the input inpedance is not the same for the XLR and RCA jacks, so things may sound different.

Almarg - Actully, Audio Research, at one time sold an active component just for converting from single ended to balanced (and the other way too). Did anyone ever buy one of these. (I think it was at the time of the LS5 preamp that only had XLRs and NO RCA jacks.)