What to use for shorting pins on balanced amp?

If you have the owners manual it should give you some guidance on which pins to short (there are three). The short loop of wire suggested by Royy should work OK, at least for a temporary solution. Just make sure you short the correct two wires. While I am not familiar with this amp, I have an Audio Research SDP-1 which also requires using a shorting plug. Without the shorting plug on the SDP-1 you will not get any signal to the main circuit through the single ended inputs (RCA) since the hot lead from the RCA jack goes direct to one pin of the balanced (XLR) connector then through another pin to the main circuit. The shorting plug makes the connection between those two pins on the XLR connector permitting signal to pass on to the circuit. I would presume your amp is wired the same. If all else fails, call Audio Research. They are very helpful for these kinds of "problems".

Any piece of solid wire between 12-22 gauge whichever fits snugly. bend it into a "U" shape. That's what I use in my Gamut amp.

usually we should plug in the short plug for RCA or pin for XLR. Coz certain power amp model may have the same output curcit for both RCA and XLR. New amp design should have the 2 separated.

For old design, amp will read signals even you are not utilizing the plug. And thats create the hum sound.

Krell and Meridian are typical brands which need the shorting plugs.