Actually, at the risk of putting the OP right off the idea of feeding a balanced power amp from an unbalanced preamp, there is one more complication, and it's this:
Is pin 1 ground/earth or 0V? Depends what you mean by ground. The amplifier's metalwork should be earthed to the mains for safety (unless it's double insulated). That doesn't mean it has to be connected to 0V though. So what do you do with the little earth tag inside the XLR connector? It's there so you can make a link between the XLR's metalwork and pin 1 if you want to. But do you want to?
If you do make the link and the amplifier's casework is connected to 0V then you're making a link between mains earth and 0V. Is that a good thing or a bad thing? Depends on how the preamp is wired. Is the preamp's 0V also connected to mains earth or is it floating?
It would be nice if balanced equipment actually did live up to its goal - that you can connect anything to anything and there'll never be a problem - but things don't always work out like that.
Having said all that, I'm not claiming that there's trouble ahead for the OP. Just wire up an RCA-XLR cable and see what happens. Most likely there won't be any problem at all.