Manitunc, The reason for the general lack of balanced connectors on the ends of tonearm cables seems to be the general lack of balanced inputs on phono stages. Very few phono stages are truly balanced internally, so in most cases there is nothing to be gained by using XLR male connectors and XLR input jacks for phono. Where the phono stage is truly balanced, it is a worthwhile modification. So, I might add, if your phono stage is not electrically balanced, don't bother.
OK, lets assume your phono stage IS balanced. In that case you want to be sure that the signal from each end of the phono cartridge is being carried on conductors of equal quality. In typical RCA phono cables, the "hot" output from the cartridge is carried on a quality conductor surrounded by a shield. However, the "ground" output is often connected to the shield itself, and both are connected to the ground side of the RCA jack at the other end (the phono stage input). If you put an XLR connector on that cable, you would want to separate and isolate the shield and the lead from the cartridge from each other. This may require running another conductor inside the shield. I don't think all RCA-terminated phono cables are constructed this way, but some are.
I haven't read all the posts here, so if this is redundant, I apologize.
