if i am correct. only ones that give no buzzing are rcas??
from my experience, rcas are buzzy and shitty. unbalanced connections have buzz, balanced doesn't. i have krk rp8s which have unbalanced rcas, balanced trs, and balanced xlr. at first i just connected them to my sb audigy2 zs's main output with a y-cable to the rcas, which sounded really shitty, way too much buzz. i went and bought an m-audio delta 44 and connected that to my krks with the balanced trs's, which sounds PERFECT. so, try and use a balanced connection... unless something is broke like dodgy said.