... next project will upgrade preamp with xlr connections.
FYI, simply adding XLR connectors to the preamp will not help, because you would still be sending single-ended signals into the cables. Unless, that is, major modifications were made to the preamp to allow it to generate true balanced signal pairs for each channel. That would likely involve addition of a tube, physically large output coupling capacitors, and other circuit elements, assuming there is room to fit all of those things.
Simply adding XLR connectors, while providing them with unbalanced signals, might result in minor sonic differences compared to running long RCA cables due to differences in the cables that are used. But those differences could just as easily be for the worse as for the better, compared to running long RCA cables.
The transformer approach I suggested is far more likely to be a good solution.
Good luck. Regards,
-- Al