Well, on my big Mac amp the extra gain of the XLR circuit raised my idle hiss to an audible level. I had to pad that down to remove the hiss. So in my experience XLRs can work in some installations and not work so well in others.
Usually the XLR side, on amplifiers, has a gain around 20 and the RCA side around 26dB.
And the on the preamp side the XLRs are 6dB hotter coming out.
Which makes this an outlier.
Did it hiss even with the XPRs unplugged or the preamp turned off?