Best Interface/Speakers to Imac set up for Garageband?

Hi,

Ive had a few noise crackles whilst making songs in GB lately and wondered if my set uop could be part of the problem and/or the cables Im using.

I have an MAudio Fast track which has a USB connection to the USB line into the Imac, also the left/right Channels on the Interface (red and white) are connected directly to the left/right of my M Audio studio monitors. This to me seems like the obvious if not only way to connec these but I was wondering if there was a better way to improve sound or rather decrease the risk of any outside interference for future projects. I believe the crackle was due to a plug in called Zebrify by Zebra but still I wnat to make sure Im doing all I can to avoid probs next time.