drmike,
Well 3 ways to do it...
OPTION 1 -
NCORE is DIY anyway. You could just install RCA jacks on the back of the amp. Simply jump your #2 pin off the XLR inside, and connect to the center pin of the RCA jack, and jump your #1 pin on the XLR to your ground ring on the RCA. But you will have to also short your #1 & #3 pin on the XLR jack already existing on the NCORE.
Best way to do this in order to still have a full functioning choice between your XLR, and RCA input is to take a pin that plugs into your XLR jack on the XLR bent in a way to allow you to plug just the #3 to #1 pin. Looks like this :

Then you could always go back between XLR or RCA. And you can just use any standard RCA cables you like.
This has been done for years by manufacturers of real XLR gear in order to allow installation of RCA jacks on their gear. Because of this you might find some pins made for this, maybe call Krell or Audio research I believe they have had models like this. Also almost 100% all Mark Levinson balanced/XLR gear was made this way, but when people lost their pins that sucked if they wanted to go back to RCA!
Or you could install a little DC flip switch too if you wanted mounted to your back panel and this would simply flip on and off being wired between your #1 and #3 pin on the XLR chassis jack so you can short and un-short it depending if your running RCA or not, but you have seen this too on many amps where they have a switch stating RCA Up, XLR Down etc... Its just simply connecting and disconnecting those 2 pins to short out the XLR.
Another way around the jumper pin is to take a pair of just plain XLR jacks not connected to cables, and run a wire from #3 to #1 and simply plug them in bare on the back of the amp going nowhere.
OPTION 2 -
Buy an adaptor plug like from CARDAS.. Expensive, and don't like them, but people use them. Which of course all Cardas does here is inside the connector assembly is short out the #1 and #3 pin together connecting them both to the ground ring on the RCA.
http://www.moon-audio.com/cardas-rca-xlr-adapter.htmlOPTION 3 -
Build custom cables... In this case you can chose either a standard Coax cable shielded which most RCA cables are made out of anyway and are 2 wires, or XLR true balanced 3 wire cables.... It does not matter.
But it comes down to which way you go, because then you need to buy a pair of RCA or XLR jacks that will be needed on the other end. The same topology applies and you will need to short the PIN #1 & #3 in the XLR end of the cable either in the connector with a little jumper loop, or using the 2 extra wires in you full XLR cable.
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OPTION 1 will of course force you to drill your chassis to fit RCA jacks, and / OR DC switch if you don't like a pin, both do the exact same function.
But whats nice about this modification is your amp is now 100% compatible to every cable and system configuration on the planet permanently. And RCA jacks only cost a few bucks each with a couple small jumper wires.