When you are dealing with components that have XLR jacks and are not fully balanced, you probably won't hear a difference either way. Some people do claim that using the rca jacks do sound a little better. The difference, however is very small. I've tried it myself and couldn't hear the difference.
With regards to using both connections, I'm not sure. I think it would depend on how the components are designed. If it were me, I would just try it and see how it sounds. I can't imagine that it would harm anything by testing it. It wouldn't hurt to call the manufacturer and see what they say.
