The line-out connectors from the tube preamplifier are not balanced for use with XLR connectors and I assume the input sockets on the CA100 amplifier have not been properly modified to accept a balanced input either, so what part of your system uses XLR connectors, which usually means a balanced line?
I guess the amplifier has been modified, since you say there are additional resistors between an XLR socket and original RCA input sockets on the amplifier but it would not make sense to change to a proper balanced 600 ohm line, just for the sake of it. I'm guessing XLR cables are for looks or for when the amplifier perhaps was used with another preamp. that
did have balanced outputs.
Personally, I would remove the XLR adaptor wiring and use both units as designed, with RCA connectors. You probably wouldn't like to pay ($$$) for a transformer to adapt the PV10 to a real balanced line anyway.