my interface sounds like its on life support when plugging to my guitar amp

[timcrow420]

So ive been using a rinky little 8 inch bass speaker for my output (using the headphone jack)

today I finally baught a expensive audio cable so I could hook it up to my guitar half stack which is six 12'' speakers made by crate.

I hooked the new audio cable to the back of the M-audio interface (red and white moniter input) and connected the other in to the red and white on my guitar amp

as soon as I got it hooked up, it makes constant long beeping sound like when someone flat lines in the hospital, however it still does play the audio, but the buzzing is still very noticable, what could cause such a thing? is it fixible or are you not able to use a guitar half stack as a studio monitor?