Sound soft/noisy when over 12V in car

This Mark 1 working good at home and in car with engine off.

But when i start the car and get +14v the sound is very soft and noisy. (Also when i start the car first and then turn in the empeg)
When i stop the car it only seems to get normal again when the power has dropped to 12v.

The empeg should be able to handle upto 16v, right?

Gonna try to buy a dc/dc convertor tommorow, and see if that helps.

Oh and i'm missing these, is that a (or the) problem?

Solved it!

Improvised some grounding leads from the shroud of the rca plugs at the amp end, and that solved it

Yup, exactly like the FAQ entry that you linked said.