I'm not sure what speakers the parking assist is supposed to go through.
A few others of us have noticed the static too. I'm still not sure what the solution to it will be. It may be coming from the PAC harness, or it may be coming from the radio, but is more pronounced now that you have better speakers and higher gains on the amplifier. I'll be doing some experimenting later this year to see if I can make it go away.
What kind of voltage are the input sections of the amplifiers seeing with the stock headunit setup?
This could be a case where a balanced line driver is actually warranted.