I had some crappy 6x9's in the rear and 5 1/4 in the front. And my radio worked fine and would go up pretty loud. I put in all new pioneer speakers, and now when I turn it up the louder it gets the more the radio will go dim and then finally shut off. It is an older Kenwood, but does my problem ly in the head unit or somewhere else. Thanks.
Could be a few things. First thing that came to my mind was a bad ground somewhere. Also possibly wires are touching or a bad fuse. Thats what I would start with.
Your head unit is only made/fused to make so much power. These pioneers could have a lower sensitivity than your older ones. And while they can and will get louder on an amp, they wont on the ~20 or so watts your head unit can give them. What is the max volume on the HU? And how high do you turn it before it starts having the problem.
This is an obligatory post.
But check the ground at the deck.