Riceroni9,
That's actually what I was doing in the beginning. I did the same thing as you by placing a headphone adapter in the headphone jack to cut off the internal speakers while using external speakers.. However here's where I ran into a problem with that.

The majority of the noise I heard from my 2000's speakers came from the tweeters. Ther first speaker set I used was a PC speaker system. It was two satalite speakers with a 25 watt subwoofer. The satalites do not have tweeters. So this noise was greatly reduced. However I compromised sound big time without having the tweeters (it just didn't sound the same). I then hooked up the board to my stereo system. My stereo has 50 watt speakers with woofer, mid, and tweeters. Using those speakers the noise was back because of the tweeters in the cabinets.

So I've hit a wall on this. I bought a AC line filter that didn't solve the problem. I then bought an inexpensive adapter that would allow me to bypass my homes grounding, and that didn't work either.

Squeak