Monoprice sells perfectly good quality XLR cables in various lengths. While not a make or break issue for sound quality, if I had XLR connections available on all my end points, I would go ahead and use them over RCA connections. But, like I said, not make or break issue there.
Your amp should still have + and - connections which go out to the speakers. That's different than the audio inputs which have unified connectors with RCA and XLR. RCA has two cables (signal/ground) while XLR has three cables (signal +, signal -, and ground) in them.