You should just run long length of RCA. The XLR will not provide ANY special shielding because it is not acting as a balanced cable with a simple RCA to XLR adapter. If you want to use XLR as a true balanced line, you have to use a balanced converter such as an ART DTI box, which converts unbalanced RCA to true balanced XLR. This box does not add any gain, btw. If you need to add gain, use a Samson S Convert box. This changes to balanced and adds adjustable gain to feed the pro amp properly if your receiver has insufficient output voltage to do so properly.
-Chris