Such a direct connection type using cables/converted cables is not recommended as you will not only encounter a hum in the sound but the signal will be sent too weak using unbalanced.
Your best choice is to get a small pre-amped 6 channel mixer to re-amplify the sound.
Plug the RCAs from the RMX-500 output to the mixer channel input
Get a master out from the mixer using XLR female / to XLR male to speaker
Hope this helps.