"there is buzzing and harmonics in the speaker output at low volumes."
"It is okay at higher volumes."
It sounds a little like a ground loop and this may be the smoking gun
"The only reason I don't use the RCA output is that it runs directly to another amp/system and the RCA cable is far longer than the XLR->RCA cable.
If the other >>"amp system"<< is in another room ... it is probably plugged into an outlet that is fed from the opposite side BUS in your main panel box that your main system's outlet is fed from
Main system and preamp plugged into outlet fed from BUS A ... other amp system in the other room plugged into outlet fed from BUS B ...this will create a ground loop
Try disconnecting the RCAs that feed the "other system" in the other room from your preamp and see if the noise goes away