Agree with Soundhound here. Buy yourself a proper cable with good sheilding and you will have no issue with a 15 foot run. Try the sub-woofer RCA cable from Parts Express, like this one, http://www.partsexpress.com/pe/showdetl.cfm?&DID=7&Partnumber=181-644
they have 12 and 25 footers.

I use two of the twelves and have not issues at all.

The other reason for balanced in professional installs is so that they can run lower voltage signals over the longer lenghts. So for single ended, your voltage is much higher and therefore your noise immunity is higher as well, and you will notice how RCA cables usually use bigger conducting wire partly for this reason.