I have read that when not using the balanced XLR inputs the XLR inputs should be jumped between pins 1 & 3, which makes the amp outputs invert the signal.That is not a correct interpretation.
When the amp is provided with balanced inputs via the XLR connectors, the two signals in the balanced signal pair for each channel (which have opposite polarities) are applied to XLR pins 2 and 3, with ground being pin 1. And the amp responds to the difference in voltage that exists at any instant of time between pins 2 and 3.
When the RCA inputs are used, the input signal that is provided on the RCA center pin for each channel is almost certainly connected directly to pin 2 of the corresponding XLR connector, as I had mentioned. And that signal is presumably sent into the same differential receiver circuit that is used to receive balanced XLR inputs. So the jumper is used to provide zero volts (i.e., ground) to the input of that circuit which would receive the signal from XLR pin 3 if the XLR input was being used. Otherwise that input would be in an uncontrolled state, and prone to noise issues.
That has no direct relation to whether or not the output of the amp is inverted relative to the input.
Regards,
-- Al