using osciloscope to set amp gains

The crests not flattening out is because your preamp is simply not clipping.  It's possible the preamp has an anti-clipping circuit, especially if it goes up to a certain voltage and stays there even if the volume control has more gain to go.  The spike (and all the other noise you describe) is either because the head unit is producing a very dirty signal or because the source CD is poorly recorded.  The trace getting "thick" is actually lots of noise (distortion) and what you're seeing is signal instability (fluctuation around the fundamental frequency) caused by the electronics in the preamp becoming overloaded.Support the12volt.com

strassehase3 wrote: thnx for the help DY, heamphyst, aleckxs, alpine guy.

Glad we could get you fixed up! It's what we do! It all reminds me of something that Molière once said to Guy de Maupassant at a café in Vienna: "That's nice. You should write it down."