The inputs on this commercial power amp has Balanced XLR inputs. They
are the 3 prong connectors like on modern microphones. But there is also
a barrier strip with 6 screws. (3 per channel). Each channel has GROUND,
HIGH and LOW.

To connect a short distance cord with RCA plugs from a receiver preamp
output, I know to use the GROUND for the outer shield, but do I use the
HIGH or the LOW for the signal IN?

I know the XLR is used for long distance cables, such as the "snake"
used by musical bands and theater systems. But I am not running that
kind of system, and the receiver output is simply an unbalanced short
run of cable (about 2 or 3 feet long).

I suppose I could just use the trial and error method without harming
anything, but I thought I'd just ask and do it right.

Thanks